Marijuana & Driving Mega Thread

[Uzi578]

New Accident Studies Confirm:
Marijuana a Lesser Driving Hazard than Alcohol


Drivers Testing Positive in Urine No More Dangerous Than Drug-Free Drivers

Drivers who test positive for marijuana in urine are no more likely to cause accidents than drug-free drivers, according to a study led by Dr. Carl Soderstrom [01] at the University of Maryland School of Medicine (2005). The study examined 2,405 drivers hospitalized in automobile accidents from 1997 through 2001.

   The study investigated the circumstances of each accident to assess which drivers were at fault or culpable. Drivers testing positive for marijuana were found to have no greater culpability than drug-free drivers.   In every age group, alcohol was the drug most strongly associated with crash culpability.  Cocaine users also showed higher crash culpability, especially in the age range of 21-40.

   Significantly, marijuana-using drivers aged 41 to 60 were statistically less likely to be at fault for accidents than drug-free drivers.   Similar results have been suggested in other studies, perhaps because marijuana-using drivers tend to slow down.

   ≥This flatly refutes the rationale for random testing of truck and bus drivers,≤ observed California NORML coordinator Dale Gieringer, ≥Urine tests for marijuana donπt reflect driving impairment.≤   Two other studies [2] have failed to find higher accident rates for drivers testing positive for marijuana in urine.

   An essential reason for these negative results is the reliance on urine tests  to detect marijuana.   Urine tests are poor indicators of impairment because they detect non-psychoactive metabolites that stay in the system for days after marijuana is smoked.  Blood tests, which measure the presence of psychoactive THC, provide a better indication of current impairment.  Usually they register positive for only a few hours after smoking, though low levels of blood THC may persist in heavy users for a day or two. Some - though not all - studies have found higher accident rates for drivers testing positive for THC in  blood.   

In another new accident study [3] - the largest yet - French researchers examined 10,748 drivers involved in fatal crashes for traces of drugs and alcohol in blood.  The study found that the presence of THC in blood was associated with a somewhat higher risk of responsibility for accidents, but significantly less so than alcohol.  The increased risk for THC was dose-dependent, ranging from 1.6 at trace levels to 3 at the highest levels (above 5 nanograms THC per milliliter of blood).  In contrast, for alcohol the risk ranged from 3 at the lowest levels (below .05% blood alcohol) to over 40 at the highest levels.   The study has proved embarrassing for drug warriors in the French government, who had prematurely rushed to pass a ≥zero tolerance≤ DUI law barring any blood traces of THC before the study was complete.  The study showed that driving with THC in blood was in fact no riskier than driving at blood alcohol levels below .05%, which is legally permitted in France.   The U.S. allows alcohol levels up to .08%.

   Numerous other studies have found that marijuana is a significantly lesser driving hazard than alcohol.   ≥Current scientific evidence shows clearly that a zero tolerance standard for THC in blood is unjustified,≤ argues California NORML coordinator Dale Gieringer. An expert panel report chaired by Dr. Franjo Grotenhermen [4] of the International Association for Cannabis Medicine concluded that levels above 3.5 to 5 nanograms per milliliter of THC in blood are generally indicative of impairment. On the other hand,  there is considerable  evidence indicating that lower levels of blood THC can be consistent with safe driving.

REFERENCES

 (1 ) Carl Soderstrom et al, ≥Crash Culpability Relative to Age and Sex for Injured Drivers Using Alcohol, Marijuana or Cocaine,≤ 49th Annual Proceedings of the Association for the Advancement of Automotive Medicine, Sept. 13-14, 2005.

(2) SR Lowenstein  and J Koziol-McLain "Drugs and traffic crash responsibility: a study of injured motorists in Colorado," J Trauma 50(2):313-30 (2001); and KLL Movig et al, "Psychoactive substance use and the risk of motor vehicle accidents" [in the Netherlands], Accident Analysis and Prevention 36: 631-6 (2004).

(3) Bernard Laumon et al, ≥Cannabis Intoxication and fatal road crashes in France: population based case-control study,≤ British Medical Journal doi:10.1136/bmj.38648.617986.1F, Dec. 2, 2005.

(4)  Franjo Grotenhermen et al., ≥Developing Science-Based Per Se Limits for Driving under the Influence of Cannabis: Findings and Recommendations by an Expert Panel,≤ (International Association for Cannabis Medicine, 2005) posted at www.canorml.org/healthfacts/DUICreport.2005.pdf.

~~From The Marijuana Mission homepage.  Link to article

The following information was taken from here.  I just got the posted all the original articles separately so it looks cleaner.  **Edit, not all info comes from here.

[Uzi578]

NORML Questions Tactics Behind Feds' Latest
"Drugged Driving" Campaign*

Source: NORML
November 29, 2005

Organization Reiterates Stance Against Driving While Impaired By Cannabis

Washington, DC: Recent allegations by the White House Office of National Drug Control Policy (ONDCP) that cannabis is a significant causal factor in on-road accidents and may adversely impact psychomotor skills up to 24 hours after past use are not supported by scientific evidence, said NORML Executive Director Allen St. Pierre. His remarks were in response to a White House campaign launched earlier this week, entitled "Steer Clear of Pot."

While acute cannabis intoxication has been shown to have demonstrable impact on psychomotor performance, these effects are typically mild and short-lived lasting at most one to three hours, and certainly not 24 hours, as claimed by the ONDCP. Moreover, unlike with alcohol, the accident risk caused by cannabis - particularly among those who are not acutely intoxicated - is often limited because subjects under its influence are generally aware of their impairment and compensate to some extent, such as by slowing down and by focusing their attention when they know a response will be required. This response is the opposite of that exhibited by drivers under the influence of alcohol, who tend to drive in a more risky manner proportional to their intoxication.

According to an analysis of on-road crashes released in September by an international expert panel: "The most meaningful recent culpability studies indicate that drivers with THC concentrations in whole blood of less than 5 ng/ml have a crash risk no higher than that of drug-free users." THC blood levels typically fall below 5 ng/ml in recreational cannabis users within 60 to 90 minutes after inhalation.

Nevertheless, St. Pierre reaffirmed NORML's stance that operating a motor vehicle under the influence of any controlled substance is unacceptable. "Responsible cannabis consumers never operate a motor vehicle in an impaired condition, regardless of whether that impairment is due to alcohol, cannabis or some other intoxicant or prescription medication," he said. "Public safety demands not only that impaired drivers be taken off the road, but that better objective measures of impairment be developed to more accurately identify drivers under the influence of drugs."

For more information, please contact either Allen St. Pierre, NORML Executive Director, or Paul Armentano, NORML Senior Policy Analyst, at (202) 483-5500. A comprehensive review of cannabis' impact on driving appears in NORML's report, "You Are Going Directly to Jail: DUID Legislation: What It Means, Who's Behind It, and Strategies to Prevent It," available online at:
http://www.norml.org/index.cfm?Group_ID=6492

~~Link to article

[Uzi578]

Paranoid Pot Smokers 'Drive More Carefully'

By Shane Holladay, Edmonton Sun
Source: Edmonton Sun
May 29, 2003

Marijuana advocates say pot could cut down on the maddening number of aggressive drivers on Alberta roads. An April 2003 Canada Safety Council study suggests Alberta has the greatest number of aggressive drivers in Canada, with 89% of surveyed drivers in Wild Rose Country admitting to at least one aggressive driving act in the past year.

"Pot people are more paranoid, so they drive more carefully," said B.C. Marijuana Party president Marc Emery of Vancouver.

Emery said he's been smoking pot and driving for 25 years, and, if anything, cannabis mellows out a driver.

"One of the real differences is that you're not goal oriented, you enjoy the ride. You're not anxious, you're not aggressive."

The survey, conducted by Thompson Lightstone & Co., includes 1,001 Canadian residents, 18 years old and older. The 89% Alberta figure compares to 88% in Ontario and 77% in Quebec and Atlantic Canada.

Dr. Alison Smiley, a University of Toronto professor who has studied how marijuana affects drivers, said people who've been smoking pot drive more like senior citizens.

"It's like an older person. They slow down on the freeway, take longer to make driving decisions," she said. "Where they can compensate for the effects they perceive, they do so by slowing down."

But Smiley stresses drivers high on pot alone are impaired. Unable to handle as much information, they may miss a pedestrian suddenly stepping out onto the road, she said.

But there's no evidence cannabis contributes to higher accident rates, said Eugene Oscapella of the Canadian Foundation for Drug Policy. "It impairs you, there's no doubt about it, but when you look at the studies themselves, there's no increase in accidents and that's the bottom line," he said.

Ottawa introduced new legislation Tuesday decriminalizing possession of small amounts of pot, sparking concerns over an increase in marijuana-impaired driving.

Federal Justice Minister Martin Cauchon said yesterday drug-impaired driving laws will also need an overhaul in light of the new regime.

"We will come up with reform on that side as soon as we can," Cauchon told the House of Commons. "We want to make sure that we will be able to develop a test that will be accepted by the courts."

Source: Edmonton Sun (CN AB)
Author: Shane Holladay, Edmonton Sun
Published: Thursday, May 29, 2003
Copyright: 2003 Canoe Limited Partnership
Contact: letters@edm.sunpub.com
Website: http://www.fyiedmonton.com/htdocs/edmsun.shtml

~~Link to article

[Uzi578]

Positive Marijuana Result Not Associated With Auto Crash Culpability

Source: NORML
September 29, 2005

Baltimore, MD: Marijuana use, as indicated by the presence of cannabis metabolites, is not associated with crash culpability among injured drivers, according to data presented at the annual conference of the Association for the Advancement of Automotive Medicine.

Researchers at the University of Maryland's National Center for Trauma and EMS obtained clinical toxicology reports for more than 2,500 injured drivers to identify the presence of alcohol, cocaine, and marijuana. Authors found that drivers who tested positive for alcohol in the blood had" significantly higher crash culpability" than sober drivers. Authors further found a "significant association" between cocaine use and crash culpability for male drivers between 21 and 40 years of age.

"In contrast, for both men and women, [the] study did not find an association between crash culpability and marijuana use," researchers determined. Drivers between the ages of 41 and 60 who tested positive for marijuana were less likely to be culpable than drug-free drivers, they added.

Because researchers based their analysis on the presence of drug metabolites in the urine rather than the presence of controlled substances in blood, authors could not determine whether the drivers' drug use directly preceded their injury or had taken place days earlier.

"While the current study does not provide evidence of cocaine and marijuana impairment, or use at the time of injury, it provides information about culpability relative to users of cocaine and marijuana," authors concluded. "To clarify the role of marijuana use in crash culpability, a large study of injured patients treated in acute care settings using blood tests to assess for marijuana use proximal to time of injury would be quite useful."

According to an analysis of on-road crashes released earlier this month by an international expert panel: "The most meaningful recent culpability studies indicate that drivers with THC concentrations in whole blood of less than 5 ng/ml have a crash risk no higher than that of drug-free users. The crash risk apparently begins to exceed that of sober drivers as THC concentrations in whole blood reach 5-10 ng/ml." Authors added, however, "Because recent studies involved only a few drivers with THC concentrations in that critical range, a reliable assessment of the associated crash risk is still lacking."

THC blood levels typically fall below 5 ng/ml in recreational cannabis users within 60 to 90 minutes after inhalation.

~~Link to article

[Uzi578]

Alcohol Impairs Driving More than Marijuana

By Arran Frood
Source: New Scientist
March 20, 2002

A single glass of wine will impair your driving more than smoking a joint. And under certain test conditions, the complex way alcohol and cannabis combine to affect driving behaviour suggests that someone who has taken both may drive less recklessly than a person who is simply drunk.

These are the findings of a major new study by British transport researchers. The unpublished research, seen exclusively by New Scientist, stops well short of condoning driving under the influence of even small amounts of cannabis.

But in a week which has seen renewed debate in Britain surrounding the criminalisation of cannabis, it throws an uncomfortable spotlight on a problem confronting governments everywhere - how to deter the growing numbers of cannabis users from "dope driving".

At present there is no accurate test that can reveal whether a driver has taken cannabis before driving, and developing one will not be easy. But even when this problem is cracked, another will remain - where to set the safety threshold for smoking cannabis.

Advocates of zero tolerance say there should be penalties for drivers caught with any amount of recently smoked cannabis in their body. The new research suggests that would only be credible if governments also adopted zero tolerance on drink driving.

Middle of the road

The new study was undertaken by the Transport Research Laboratory in Crowthorne, Berkshire, and confirms the results of a preliminary study more than a year ago. Researchers at the TRL, led by Barry Sexton, gave 15 volunteers doses of cannabis or alcohol, or a combination of both, before letting them loose on an array of psychomotor tests and a sophisticated driving simulator.

The volunteers were given either enough alcohol to raise alcohol levels in the blood to 50 milligrams per 100 millilitres - about 60 per cent of Britain's legal limit of 80 mg/100 ml - or a specially prepared marijuana joint designed to deliver the same high typically experienced by smokers.

In the study, cannabis significantly affected only one criterion, known as tracking ability. Volunteers found it more difficult to hold a constant speed and follow the middle of the road accurately while driving around a figure-of-eight loop. The TRL researchers point out in their draft report that this test requires drivers to hold their concentration for a short time, a task which is particularly badly affected by the intoxicating effects of cannabis.

Cautious driving

However, volunteers drinking the equivalent of a glass of wine fared worse than those who had smoked a joint. Those who were given both alcohol and cannabis performed worse still, reinforcing the idea that alcohol has a cumulative effect when taken with other drugs.

But the study also found that drivers on cannabis tended to be aware of their intoxicated state, and drove more cautiously to compensate. Indeed, doped-up volunteers often rated themselves as being more impaired than police surgeons brought in to evaluate their sobriety.

Surprisingly, drinking alcohol didn't offset this cautious behaviour, opening up the unproven possibility that a driver who is moderately drunk might be better off under some conditions if they had also smoked.

This cautious behaviour is in line with findings by other researchers. "Whereas alcohol promotes risk taking like fast speeds and close following, cannabis promotes conservative driving, but may cause attention problems and misperceptions of time," says Nicholas Ward, technical adviser to the Immortal project - a three-year European Union trial designed to quantify the crash risk drivers face after taking various drugs and medicines.

~~Link to article

[Uzi578]

Cannabis May Make You a Safer Driver

By Jonathon Carr-Brown 
Source: The Sunday Times
August 13, 2000

Taking the high road may not be so dangerous after all. Ministers are set to be embarrassed by government-funded research which shows that driving under the influence of drugs makes motorists more cautious and has a limited impact on their risk of crashing.

In the study, conducted by the Transport Research Laboratory, grade A cannabis specially imported from America was given to 15 regular users. The doped- up drivers were then put through four weeks of tests on driving simulators to gauge reaction times and awareness.

Regular smokers were used because previous tests in America using first- timers resulted in the volunteers falling over and feeling ill. The laboratory found its guinea pigs through what it described as a "snowballing technique" - one known user was asked to find another after being promised anonymity and exemption from prosecution agreed with the Home Office.

Instead of proving that drug-taking while driving increased the risk of accidents, researchers found that the mellowing effects of cannabis made drivers more cautious and so less likely to drive dangerously.

Although the cannabis affected reaction time in regular users, its effects appear to be substantially less dangerous than fatigue or drinking. Research by the Australian Drugs Foundation found that cannabis was the only drug tested that decreased the relative risk of having an accident.

The findings will embarrass ministers at the Department of the Environment, Transport and the Regions (DETR) who commissioned the study after pressure from motoring organisations and anti-drug campaigners. Lord Whitty, the transport minister, will receive the report later this month.

Last week police revealed details of new drug-driving tests to be administered by the roadside, which were received with some amusement. They require suspected drug- drivers to stand on one leg, lean back and touch their nose with their eyes closed, and to count to 30 silently with their eyes shut. This is apparently difficult for those on a drug trip.

The advertising company McCann-Erickson has already prepared a television campaign using Pulp's song Sorted for Es and Whizz, the slogan "Never drive on drugs" and the pay-off line "then you come down".

However, if the findings are less than frightening on the effects of marijuana, they may convince ministers to put more money into raising driver awareness of fatigue. Tiredness is now blamed for causing 10% of all fatal accidents, compared with 6% for alcohol and 3% for drugs.

A low-key radio campaign will be launched tomorrow warning drivers to take breaks.

The report's surprising conclusions will not sway organisations such as the RAC, which believes there is incontrovertible evidence that drug-driving is a growing menace. DETR statistics published in January showed a six-fold increase in the number of people found to be driving with drugs in their system after fatal road accidents. The figure jumped from 3% in 1989 to 18%.

Dr Rob Tunbridge, the report's author, refused to reveal his findings before they were published but said: "If you were to ask me to rank them in order of priority, fatigue is the worst killer, followed by alcohol, and drugs follow way behind in third."

Tunbridge admitted that the effect of drugs differed with the individual, the amount taken, the environment they were taken in and the point at which you tested reactions.

Cocaine users are known to be alert drivers when they first take the drug, but then they have a tendency to fall asleep at the wheel. The particular problem with cannabis is that it stays in a person's system for up to 30 hours but its effects wear off within a few hours.

E-mail: jonathon.carr-brown@sunday-times.co.uk

Published: August 13, 2000
Copyright 2000 Times Newspapers Ltd.

~~Link to article

[Uzi578]

Marijuana Use and Driving

By Robbe HWJ
November 1994

    * Abstract
    * Introduction
    * General procedures
    * Pilot Study
    * Pilot Study Results
    * Study 1: Driving on a restricted highway
    * Study 1 Results
    * Study 2: Driving on a normal highway in traffic
    * Study 2 Results
    * Study 3: Driving in urban traffic
    * Study 3 Results
    * Discussion
    * References

Abstract

Abstract: This article concerns the effects of marijuana smoking on actual driving performance. It presents the major results of one laboratory and three on-road driving studies. The latter were conducted on a closed section of a primary highway, on a highway in the presence of other traffic and in urban traffic, respectively. This program of research has shown that marijuana produces only a moderate degree of driving impairment which is related to the consumed THC dose. The impairment manifests itself mainly in the ability to maintain a steady lateral position on the road, but its magnitude is not exceptional in comparison with changes produced by many medicinal drugs and alcohol. Drivers under the influence of marijuana retain insight into their performance and will compensate where they can (e.g., by increasing distance between vehicles or increasing effort). As a consequence, THC's adverse effects on driving performance appeared relatively small in the tests employed in this program.

Introduction

After alcohol, delta-9-tetrahydrocannabinol (THC), marijuana's major psychoactive constituent, is the drug which is found most often in the blood of drivers involved in road accidents. With some exceptions, epidemiological studies indicate the presence of THC in roughly 4-12% of drivers injured or killed in traffic accidents: for example, 10% in New York (Terhune 1982), 7% in New South Wales (Chesher and Starmer 1983), 8% in North Carolina (Mason and McBay 1984), 11% in DŸsseldorf (Daldrup et al. 1987), 10% in Tasmania (McLean 1987) and 11% in Ontario (Cimbura et al. 1990). The most recent data regarding the incidence of drugs in fatally injured drivers in the United States are available from a nationwide study conducted in 1990 and 1991 (Terhune et al. 1992): THC was found in only 4% of the drivers. This relatively low percentage may indicate a declining trend in the incidence rate of THC in fatally injured drivers in the United States, explainable by the declining prevalence of marijuana use throughout the 1980s (Johnston et al. 1992).

These numbers are, however, inconclusive regarding THC's contribution to accidents because alcohol has been a severe confounding factor in all surveys of injured or killed drivers: generally 60-80% of drivers who were found positive for THC also showed the presence of alcohol. Another problem of these surveys is the common lack of sound control groups. To determine whether drivers under the influence of THC are overrepresented in accidents, the THC incidence in accident victims should be compared to the THC incidence in randomly selected drivers passing the same accident site at the same times and days of the week. This has been done for alcohol (Borkenstein et al. 1974), but not for THC.

The lack of separate control groups has been circumvented by the use of a 'culpability index'. This index is the ratio of the percentage of drivers with detectable drug levels and deemed responsible for a traffic accident to that of drug-free drivers from the same sample who were likewise culpable. But results from this approach have not been consistent: three studies yielded a culpability index of about 1.7 (Warren et al. 1981, Terhune 1982, Donelson et al. 1985), two other studies failed to find a significantly elevated culpability index for marijuana users (Williams et al. 1985, Terhune et al. 1992). For this and other reasons given above, the independent contribution of THC to accidents remains exceedingly obscure. Several literature reviews, the most recent by Robbe (1994), have shown that the results from driving simulator and closed-course tests indicate that THC in inhaled doses up to 250 µg/kg has relatively minor effects on driving performance, certainly less than blood alcohol concentrations (BACs) in the range 0.08-0.10 g%. In contrast to this, laboratory studies have repeatedly shown performance impairment occurring after inhaled doses as low as about 40 µg/kg. These became large and persistent after 100-200 µg/kg doses. Tracking, divided attention and vigilance test performance were particularly vulnerable to THC's effects. Assuming that both sets of results are valid for the particular circumstances under which they were obtained, they demonstrate that performance decrements obtained under the artificial and non-life threatening conditions in the laboratory do not automatically predict similar decrements in driving simulations that are closer to real-world driving. These conflicting results led, however, to opposing opinions regarding marijuana's effects on driving performance. Real-world driving studies were therefore warranted.

Only one study (Klonoff 1974) had been conducted in actual traffic before the present research program started. In a city driving study, Klonoff assessed the effects of two THC doses, 4.9 and 8.4 mg, which are equivalent to 70 and 120 µg/kg for a 70 kg person. Aspects of subjects' driving performance were scored by a professional examiner. The results showed that subjects performed less competently when under the influence of the highest, but not the lowest dose. In particular, they scored lower on judgment and concentration scales. Several investigators, however, criticized the method used by Klonoff for measuring driving performance on the grounds that the examiners' reliability was never determined and that the scoring instrument had never been shown to provide measures related to driving safety (Moskowitz 1985, Smiley 1986). Furthermore, Klonoff administered relatively low THC doses to his subjects. The effects of high doses of THC on driving in real traffic still needed to be determined.

The studies reported in this article were conducted to escape these limitations. First, the effects of low, moderate and high THC doses on highway driving were determined, both in the absence and presence of other traffic. Second, Klonoff's city driving study was replicated, with some modifications with regards to the employed procedures and with the addition of another group of subjects who undertook the same driving test but then under the influence of a low dose of alcohol.

An important practical objective of the program was to determine whether degrees of driving impairment can be accurately predicted from either measured concentrations of THC in plasma or performance measured in potential roadside 'sobriety' tests of tracking ability or hand and posture stability. The results (Robbe 1994), like many reported before, indicate that none of these measures accurately predicts changes in actual driving performance under the influence of THC.

General procedures

Subjects in all studies were recreational users of marijuana or hashish, i.e., smoking the drug more than once a month, but not daily. They were all healthy, between 21 and 40 years of age, had normal weight and binocular acuity, and were licensed to drive an automobile. Furthermore, law enforcement authorities were contacted, with the volunteers' consent, to verify that they had no previous arrests or convictions for drunken driving or drug trafficking.

Each subject was required to submit a urine sample immediately upon arrival at the test site. Samples were assayed qualitatively for the following common 'street drugs' (or metabolites): cannabinoids, benzodiazepines, opiates, cocaine, amphetamines and barbiturates. In addition, a breath sample was analyzed for the presence of alcohol using a Lion-SD3 breath-analyzer.

Subjects were accompanied during every driving test by an licensed driving instructor. A redundant control system in the test vehicle was available for controlling the car, should emergency situations arise.

Marijuana and placebo marijuana cigarettes were supplied by the U.S. National Institute on Drug Abuse. The lowest and highest THC concentrations in the marijuana cigarettes used in the studies were 1.75% and 3.57%, respectively.

Pilot study
Methods

The pilot study was conducted in a hospital under strict medical supervision to identify THC doses that recreational marijuana users were likely to consume before driving. Twenty-four subjects, twelve males and twelve females, participated. They were allowed to smoke part or all of the THC content in three cigarettes until achieving the desired psychological effect. Cigarettes were smoked through a plastic holder in a manner determined by the subjects. The only requirement was to smoke for a period not exceeding 15 minutes. When subjects voluntarily stopped smoking, cigarettes were carefully extinguished and retained for subsequent gravimetric estimation of the amount of THC consumed.

Results

Six subjects consumed one cigarette, thirteen smoked two and four smoked three. The average amount of THC consumed was 20.8 mg, after adjustment for body weight, 308 µg/kg. It should be noted that the amounts of THC consumed represent both the inhaled dose and the portion that was lost through pyrolysis and side-stream smoke during the smoking process. There were no significant differences between males and females, nor between frequent and infrequent users, with respect to the weight adjusted preferred dose. It was decided that the maximum dose for subsequent driving studies would be 300 µg/kg. This is considerably higher than doses that have usually been administered to subjects in experimental studies (typically, 100­200 µg/kg THC).

Study 1: Driving on a restricted highway
Methods

The first driving study was conducted on a highway closed to other traffic. One objective of this study was to determine whether it would be safe to repeat the study on a normal highway in the presence of other traffic. The second objective was to define the dose-effect relationship between inhaled THC dose and driving performance. The same twelve men and twelve women who participated in the laboratory study served again as the subjects. They were treated on separate occasions with marijuana cigarettes containing THC doses of 0 (placebo), 100, 200, and 300 µg/kg. Treatments were administered double-blind and in a counterbalanced order. On each occasion, subjects performed a road-tracking test beginning 40 minutes after initiation of smoking and repeated one hour later. The test, developed and standardized by O'Hanlon et al. (1982, 1986), involved maintaining a constant speed at 90 km/h and a steady lateral position between the delineated boundaries of the traffic lane. Subjects drove 22 km on a primary highway and were accompanied by a licensed driving instructor. The primary dependent variable was the standard deviation of lateral position (SDLP), which has been shown to be both highly reliable and very sensitive to the influence of sedative drugs and alcohol. Other dependent variables were mean speed, and standard deviation of speed and steering wheel angle.

Results

All subjects were willing and able to finish the driving tests without great difficulty. The study demonstrated that marijuana impairs driving performance as measured by an increase in SDLP; all three THC doses significantly affected SDLP relative to placebo. The driving performance decrement after smoking marijuana persisted almost undiminished for two hours after smoking. Marijuana's effects on SDLP were compared to those of alcohol obtained in a very similar study by Louwerens et al. (1985, 1987). It appeared that the effects of the various administered doses of THC (100­300 µg/kg) on SDLP were equivalent to those associated with BACs in the range of 0.03-0.07 g%. Other driving performance measures were not significantly affected by THC. Both the observed degree of driving impairment, and what subjects said and did, indicated that normal safeguards would be sufficient for ensuring safety in further testing. Hence, the final conclusion was to repeat this study on a normal highway in the presence of other traffic.

Study 2: Driving on a normal highway in traffic
Methods

The second driving study was conducted on a highway in the presence of other traffic and involved both a road-tracking and a car-following test. A new group of sixteen subjects, equally comprised of men and women, participated in this study. A conservative approach was chosen in designing the present study in order to satisfy the strictest safety requirements. That is, the study was conducted according to an ascending dose series design where both active drug and placebo conditions were administered, double-blind, at each of three THC dose levels. THC doses were the same as those used in the previous study, namely 100, 200, and 300 µg/kg. Cigarettes appeared identical at each level of treatment conditions. If any subject had reacted in an unacceptable manner to a lower dose, he/she would not have been permitted to receive a higher dose.

The subjects began the car-following test 45 minutes after smoking. The test was performed on a 16 km stretch of the highway and lasted about 15 minutes. After the conclusion of this test, subjects performed a 64-km road-tracking test on the same highway which lasted about 50 minutes. At the conclusion of this test, they participated again in the car-following test.

The road-tracking test was the same as in the previous study except for its duration and the presence of other traffic. Subjects were instructed to maintain a constant speed of 95 km/h and a steady lateral position between lane boundaries in the right traffic lane. They were allowed to deviate from this only if it became necessary to pass a slower vehicle in the same lane. Data from the standard test were analyzed to yield the same performance measures as in the previous study.

The car-following test involved attempting to match velocity with, and maintain a constant distance from a preceding vehicle as it executed a series of deceleration/acceleration maneuvers. The preceding vehicle's speed would vary between 80 and 100 km/h and the subject was instructed to maintain a 50 m distance however the preceding vehicle's speed might vary. The duration of one deceleration and acceleration maneuver was approximately 50 seconds and six to eight of these maneuvers were executed during one test, depending upon traffic density. The subject's average reaction time to the movements of the preceding vehicle, mean distance and coefficient of variation of distance during maneuvers were taken as the dependent variables from this.

Results

All subjects were able to complete the series without suffering any untoward reaction while driving. Road-tracking performance in the standard test was impaired in a dose-related manner by THC and confirmed the results obtained in the previous closed highway study. The 100 µg/kg dose produced a slight elevation in mean SDLP, albeit not statistically significant. The 200 µg/kg dose produced a significant elevation, of dubious practical relevance. The 300 µg/kg dose produced a highly significant elevation which may be viewed as practically relevant but unexceptional in comparison with similarly measured effects of many medicinal drugs. After marijuana smoking, subjects drove with an average speed that was only slightly lower than after a placebo and very close to the prescribed level.

In the car-following test, subjects maintained a distance of 45­50 m while driving in the successive placebo conditions. They lengthened mean distance by 8, 6 and 2 m in the corresponding THC conditions after 100, 200 and 300 µg/kg, respectively. The initially large drug-placebo difference and its subsequent decline is a surprising result. Our explanation for this observation is that the subjects' caution was greatest the first time they undertook the test under the influence of THC and progressively less thereafter. The reaction time of the subjects to changes in the preceding vehicle's speed increased following THC treatment, relative to placebo. The administered THC dose was inversely related to the change in reaction time, as it was to distance. However, increased reaction times were partly due to longer distance. Statistical adjustment for this confounding variable resulted in smaller and non-significant increases in reaction time following marijuana treatment, the greatest impairment (0.32 s) being observed in the first test following the lowest THC dose. Distance variability followed a similar pattern as mean distance and reaction time; the greatest impairment was found following the lowest dose.

Study 3: Driving in urban traffic
Methods

The program proceeded to the third driving study, which involved tests conducted in high-density urban traffic. There were logical and safety reasons for restricting the THC dose to 100 µg/kg. It was given to a group of regular marijuana (or hashish) users, along with a placebo. For comparative purposes, another group of regular alcohol users was treated with a modest dose of its members' preferred recreational drug, ethanol, or a placebo, before undertaking the same city driving test. Two groups participated, each composed of sixteen new subjects comprising equal numbers of men and women. Subjects in the alcohol group were regular users of alcohol, but not marijuana. Both groups were treated on separate occasions with the active drug and a placebo. Marijuana was administered to deliver 100 µg/kg THC. The driving test commenced 30 minutes after smoking. The alcohol dose was chosen to yield a BAC approaching 0.05 g% when the driving test commenced 45 minutes after onset of drinking. Active drug and placebo conditions were administered double-blind and in a counterbalanced order in each group.

Driving tests were conducted in daylight over a constant 17.5 km route within the city limits of Maastricht. Subjects drove their placebo and active-drug rides through heavy, medium and low density traffic on the same day of the week, and at the same time of day. Two scoring methods were employed in the present study. The first, a 'molar' approach, required the driving instructor acting as the safety controller during the tests to rate the driver's performance retrospectively using a standard scale. The second, a more 'molecular' approach, involved the employment of a specially trained observer who applied simple and strict criteria for recording when the driver made or failed to make each in a series of observable responses at predetermined points along a chosen route.

Results

The study showed that a modest dose of alcohol (BAC = 0.034 g%) produced a significant impairment in city driving, as measured by the molar approach, relative to a placebo. More specifically, alcohol impaired both vehicle handling and traffic maneuvers. Marijuana, administered in a dose of 100 µg/kg THC, on the other hand, did not significantly change mean driving performance as measured by this approach. Neither alcohol nor marijuana significantly affected driving performance measures obtained by the molecular approach, indicating that it may be relatively insensitive to drug-induced changes.

Driving quality, as rated by the subjects, contrasted with observer ratings. Alcohol impaired driving performance according to the driving instructor, but subjects did not perceive it; marijuana did not impair driving performance, but the subjects themselves perceived their driving performance as such. Both groups reported about the same amount of effort in accomplishing the driving test following a placebo. Yet only subjects in the marijuana group reported significantly higher levels of invested effort following the active drug. Thus there is evidence that subjects in the marijuana group were not only aware of their intoxicated condition, but were also attempting to compensate for it. These seem to be important findings. They support both the common belief that drivers become overconfident after drinking alcohol and investigators' suspicions that they become more cautious and self-critical after consuming low doses of THC, as smoked marijuana.

Discussion

The results of the studies corroborate those of previous driving simulator and closed-course tests by indicating that THC in inhaled doses up to 300 µg/kg has significant, yet not dramatic, dose-related impairing effects on driving performance. Standard deviation of lateral position in the road-tracking test was the most sensitive measure for revealing THC's adverse effects. This is because road-tracking is primarily controlled by an automatic information processing system which operates outside of conscious control. The process is relatively impervious to environmental changes, but highly vulnerable to internal factors that retard the flow of information through the system. THC and many other drugs are among these factors. When they interfere with the process that restricts SDLP, there is little the afflicted individual can do by way of compensation to restore the situation. Car-following and, to a greater extent, city driving performance depend more on controlled information processing and are therefore more accessible for compensatory mechanisms that reduce the decrements or abolish them entirely.

THC's effects after doses up to 300 µg/kg never exceeded alcohol's at BACs of 0.08 g% and were in no way unusual compared to many medicinal drugs (Robbe 1994). Yet THC's effects differ qualitatively from many other drugs, especially alcohol. Evidence from the present and previous studies strongly suggests that alcohol encourages risky driving whereas THC encourages greater caution, at least in experiments. Another way THC seems to differ qualitatively from many other drugs is that the former's users seem better able to compensate for its adverse effects while driving under the influence.

Although THC's adverse effects on driving performance appeared relatively small in the tests employed in this program, one can still easily imagine situations where the influence of marijuana smoking might have a dangerous effect; i.e., emergency situations which put high demands on the driver's information processing capacity, prolonged monotonous driving, and after THC has been taken with other drugs, especially alcohol. Because these possibilities are real, the results of the present studies should not be considered as the final word. They should, however, serve as the point of departure for subsequent studies that will ultimately complete the picture of THC's effects on driving performance.

References

    * Borkenstein R.F., R.F. Crowther, R.P. Schumate, W.B. Ziel and R. Zylman , 1974. The Role of the Drinking Driver in Traffic Accidents (The Grand Rapids Study). Blutalkohol 11(Suppl 1): 1-131.
    * Chesher G.B. and G.A. Starmer, 1983. Cannabis and Human Performance Skills. Drug and Alcohol Authority Research, Grant Report Series, Sydney.
    * Cimbura G., D.M. Lucas, R.C. Bennett and A.C. Donelson, 1990. Incidence and toxicological aspects of cannabis and ethanol detected in 1394 fatally injured drivers and pedestrians in Ontario (1982-1984). Journal of Forensic Sciences 35: 1035-1041.
    * Daldrup T., G. Reudenbach and K. Kimm, 1987. Cannabis und Alkohol im Strassenverkehr. Blutalkohol 24: 144-156.
    * Donelson A.C., G. Cimbura, R.C. Bennett and D.M. Lucas, 1985. The Ontario monitoring project: Cannabis and alcohol use among drivers and pedestrians fatally injured in motor vehicle accidents from March 1982 through July 1984. Traffic Injury Research Foundation of Canada, Ottawa.
    * Johnston L.D., P.M. O'Malley and J.G. Bachman, 1992. Smoking, Drinking, and Illicit Drug Use among American Secondary School Students, College Students, and Young Adults, 1975-1991. Institute for Social Research, University of Michigan, Michigan.
    * Klonoff H., 1974. Marijuana and driving in real-life situations. Science 186: 317-323.
    * Louwerens J.W., A.B.M. Gloerich, G. de Vries, K.A. Brookhuis and J.F. O'Hanlon, 1985. De Invloed van Verschillende Bloedalcoholspiegels op Objectief Meetbare Aspekten van Feitelijk Rijgedrag. Technical Report No. VK 85-03, Traffic Research Centre, University of Groningen, Groningen.
    * Louwerens J.W., A.B.M. Gloerich, G. de Vries, K.A. Brookhuis and J.F. O'Hanlon, 1987. The relationship between drivers' blood alcohol concentration (BAC) and actual driving performance during high speed travel. Pages 183-192 in P.C. Noordzij and R. Roszbach, eds., Alcohol, Drugs and Traffic Safety. Proceedings of the 10th International Conference on Alcohol, Drugs and Traffic Safety. Excerpta Medica, Amsterdam.
    * Mason A.P. A.J. and McBay, 1984. Ethanol, Marijuana, and other drug use in 600 drivers killed in single-vehicle crashes in North Carolina, 1978-1981. Journal of Forensic Sciences 29: 987-1026.
    * McLean S., R.S. Parsons, R.B. Chesterman, R. Dineen, M.G. Johnson and N.W. Davies, 1987. Drugs, alcohol and road accidents in Tasmania. The Medical Journal of Australia 147: 6-11.
    * Moskowitz H., 1985. Marijuana and driving. Accident Analysis and Prevention 17: 323-346.
    * O'Hanlon J.F., T.W. Haak, G.J. Blaauw and J.B.J. Riemersma, 1982. Diazepam impairs lateral position control in highway driving. Science 217: 79-81.
    * O'Hanlon J.F., K.A. Brookhuis, J.W. Louwerens and E.R. Volkerts, 1986. Performance testing as part of drug registration. Pages 311-330 in J.F. O'Hanlon and J.J. de Gier, eds., Drugs and Driving. Taylor and Francis, London.
    * Robbe H.W.J., 1994. Influence of Marijuana on Driving. Doctoral thesis, Institute for Human Psychopharmacology, University of Limburg, Maastricht.
    * Smiley A.M., 1986. Marijuana: On-road and driving simulator studies. Alcohol, Drugs and Driving: Abstracts and Reviews 2: 121-134.
    * Terhune K.W., 1982. The Role of Alcohol, Marijuana and Other Drugs in the Accidents of Injured Drivers. Technical Report to US Department of Transportation, Calspan Field Services, Inc.
    * Terhune K.W., C.A. Ippolito, D.L. Hendricks, J.G. Michalovic, S.C. Bogema, P. Santinga, R. Blomberg and D.F. Dreusser, 1992. The Incidence and Role of Drugs in Fatally Injured Drivers. Department of Transportation, Washington, DC.
    * Warren R.A., H.M. Simpson HM, J. Hilchie, G. Cimbura, D. Lucas and R. Bennett, 1981. Drugs detected in fatally injured drivers in the Province of Ontario. Pages 203-217 in L. Goldberg, ed., Alcohol, Drugs and Traffic Safety. Proceedings, 8th International Conference on Alcohol, Drugs and Traffic Safety. Almqvist and Wiksell International, Stockholm.
    * Williams A.F., M.A. Peat, D.J. Crouch, J.K. Wells and B.S. Finkle, 1985. Drugs in fatally injured young male drivers. Public Health Reports 100: 19-25.

~~Link to article

[Uzi578]

Alcohol + MJ Toxicology

By Jim Rosenfield
October 14, 1994

He posted an "executive summary" of a 1986 study, he did not actually conduct the study or write it.

The Interaction Between Alcohol and Marijuana
A Dose Dependent Study of the Effects on Human Moods and Performance Skills

By
Gregory B. Chesher
Helen Dauncey
John Crawford
Kim Horn

Psychopharmacology Research Unit
Department of Pharmacology
University of Sydney, NSW.

for the Federal Office of Road Safety (Australia)

Executive Summary
=================

1. A study was designed to examine the effects of marijuana and alcohol when
taken alone and in combination on human skills performance and mood.

2. Four dosage conditions were employed for each drug (placebo and three
active doses). All possible combinations of these dosage conditions were
tested (ie 16 dosage groups).

3. Twenty subjects were used for each dosage group, the experiment employing
320 subjects in all. Each subject attended the laboratory on one occasion
only.

4. Data collected were for psychomotor performance using a battery of
computer-presented tests, mood effects, subjective assessments of the nature
and degree of intoxication, and the subjective assessment of the effects of
the drugs on driving skills and willingness to drive a motor vehicle.

5. The performance battery included tests of human skills related to those
considered necessary to drive a motor vehicle with safety.

6. The population sample were recruited by advertisements on two Sydney
"Rock music" FM radio stations. All volunteers were non-naive as regards
marijuana use and were indeed heavy to very heavy users of this drug. The
extent of alcohol use by the volunteers was considered to be within the
normal range of use of this drug within the community.

7. The attitudes expressed concerning the dangers associated with the use of
the two drugs indicated that the population sample was heavily biased
against alcohol and in favor of marijuana

8. The subjective assessment of the doses of each drug employed indicated
that they were comparable. The subjects assessed the degree of intoxication
by marijuana as being of a similar intensity as that produced by alcohol.
The doses selected therefore appear to be relevant to those used within the
social experience of the volunteer population

9. Both drugs produced significant dose-dependent effects on the performance
measures, on the intoxication rating scales and on some of the mood
measures.

10. However, there were both quantitative and qualitative differences
between these effects, both on the performance measures and on the
subjective mood effects of the two drugs.

11. By far the major effects on these tests were those produced by alcohol.

12. The effect on skills performance of alcohol and marijuana when taken in
combination was essentially one of addition. Marijuana tended to increase
the intensity of the performance impairment produced by alcohol. However
there was evidence to suggest that the lowest dose of marijuana produced a
degree of antagonism of the effects of alcohol.

13. Marijuana had no effect on the absorption or metabolism of alcohol. The
blood alcohol concentration was not affected by any of the doses of
marijuana used.

14. The results of this study indicate clearly that alcohol and marijuana
are distinctly different drugs. The effects produced on the performance
measures were qualitatively and quantitatively different. In addition, the
differences in the nature of the drug-induced subjective intoxication and
the self-reported changes in mood effects such as anxiety and alertness,
strongly suggested different drug actions.

15. The ability to discriminate and assess the degree of intoxication with
alcohol was not affected by marijuana. However, the ability to assess the
intoxication due to marijuana was greatly affected by alcohol. The
subjective intoxication produced by marijuana appears to be of a more subtle
nature than that produced by alcohol.

16. Evidence is presented which suggests that under the influence of
alcohol, subjects engage in a "speed-accuracy trade-off". They are prepared
to make a hasty response to a question rather than to spend more time to
ensure a correct answer. This effect could be related to a risk-taking
behaviour. The results with marijuana on the other hand suggested a slower
and more careful approach to the problem, though as with alcohol, an
increased error rate in responses was recorded.

17. Evidence is presented which suggests that marijuana produces periodic
attentional lapses.

18. The results strongly suggest that the performance deficits and mood
changes produced by alcohol are of a greater magnitude than those produced
by marijuana.

19. Recommendations for directions of further research are made

 ----------------------

whhheeeeewwww, apologies for the long post, but at least there won't be 30
replies asking for methodology, numbers, etc etc. (email privately for
specifics!!!).  Anyway, basically mj impairs driving,  but not as much as
alcohol. As for the NTSA study, I do wonder about the methodology.  Exposure
is obviously going to be a major factor and there is little doubt that there
are more people using alcohol than mj, so of course there will be more
alcohol related accidents (or did they control for this? - I haven't read
it). I also suspect that people who have been drinking may be more likely to
drive afterwards than people using mj.  In any case legalisation of more
drugs (which I am *for* incidentally) will require cheap accurate techniques
for measurement of levels of intoxication to allow enforcement of any
restrictions (eg don't smoke and drive).  IMHO, this perhaps is one of the
few legitimate anti-legalisation arguments.

David
davids@gpo.pa.uq.oz.au

David Steadson, Senior Research Assistant
Social and Preventive Medicine, University of Queensland
Brisbane, Australia.

~~Link to article

[Uzi578]

Toxic Effects of Cannabis and Cannabinoids: Review of the Evidence

Article: Science and Technology - Ninth Report
By: Ordered by the House of Lords to be printed November 4, 1998

This contains a lot of information, not all concerning MJ and driving.  Im just posting the related information.

  4.6     Intoxication with cannabis leads to a slight impairment of psychomotor and cognitive function, which is important for those driving a vehicle, flying an aircraft or operating machinery (DH Q 197). The Department of Health rate this as "the major concern from a public health perspective" raised by recreational use (p 46), and Professor Hall considers it the most serious possible short-term consequence of cannabis use, both for the user and for the public (p 222).

  4.7     There is some disagreement about how long such impairments persist after taking cannabis: most assume that they last for only a few hours (e.g. Kendall p 266); but Professor Heather Ashton of the University of Newcastle-upon-Tyne, principal author of the BMA report, suggested that subtle cognitive impairments could persist for 24 or even 48 hours or more (Q 72), whereas the DETR say "probably .... 24 hours at most" (Press Notice 94/Transport, 11 February 1998). On the other hand the impairment in driving skills does not appear to be severe, even immediately after taking cannabis, when subjects are tested in a driving simulator. This may be because people intoxicated by cannabis appear to compensate for their impairment by taking fewer risks and driving more slowly, whereas alcohol tends to encourage people to take greater risks and drive more aggressively (POST note 113; cp DH p 240).

  4.8     Analysis of blood samples from road traffic fatalities in 1996-97 (the results of the first 15 months of a three year DETR study—Press Notice 94/Transport, 11 February 1998) showed that 8 per cent of the victims were positive for cannabis, including 10 per cent of the victims who were driving. However, it is not clear what figures would have been obtained from a random sample of road users not involved in accidents (DH Q 211); and some of those who tested positive may have taken the cannabis as much as 30 days before, so that the effects would have worn off long since (DH p 240). The interpretation of traffic accident data is further confounded by the fact that 22 per cent of the drivers found to be cannabis­positive also had evidence of alcohol intake; proportions of alcohol­positives among cannabis­positive drivers as high as 75 per cent have been reported in other countries in similar studies. Professor Hall considers cannabis's contribution to danger on the roads to be very small; in his view the major effect of cannabis use on driving may be in amplifying the impairments caused by alcohol (cp Keen Q 42). According to a survey of 1,333 regular cannabis users by the Independent Drug Monitoring Unit (IDMU) in 1994, users who drove reported a level of accidents no higher than the general population; those with the highest accident rates were more likely to be heavier poly-drug users.

  4.9     It is difficult to see how cannabis intoxication could be monitored, if its use were permitted. There could be no equivalent of the breathalyser for alcohol, since small amounts of cannabis continue to be released from fat into the blood long after any short-term impairment has worn off (see paragraph 3.5 above).

~~Link to article

[Uzi578]

Marijuana Myths, Claim #12:
Marijuana is a Major Cause of Highway Accidents

The detrimental impact of alcohol on highway safety has been well documented. Marijuana's opponents claim that it, too, causes significant impairment and that any increase in use will lead to increased highway accidents and fatalities.

THE FACTS

In high doses, marijuana probably produces driving impairment in most people. However, there is no evidence that marijuana, in current consumption patterns, contributes substantially to the rate of vehicular accidents in America.

A number of studies have looked for evidence of drugs in the blood or urine of drivers involved in fatal crashes. All have found alcohol present in 50% or more. Marijuana has been found much less often. Furthermore, in the majority of cases where marijuana has been detected, alcohol has been detected as well. 77

    For example, a recent study sponsored by the U.S. National Highway Traffic Safety Administration (NHTSA) involving analysis of nearly 2000 fatal accident cases, found 6.7 % of drivers positive for marijuana. In more than two-thirds of those, alcohol was present and may have been the primary contributor to the fatal outcome. 78

To accurately assess marijuana's contribution to fatal crashes, the positive rate among deceased drivers would have to be compared to the positive rate from a random sample of drivers not involved in fatal accidents. Since the rate of past-month marijuana use for Americans above the legal driving age is about 12 percent, on any given day a substantial proportion of all drivers would test positive, particularly since marijuana s metabolites remain in blood and urine long after its psychoactive effects are finished.

    A recent study found that one-third of those stopped for "reckless driving" between the hours of 7 p.m. and 2 a.m. - mostly young males - tested positive for marijuana (and no other drugs).79 To be meaningful, these test results would have to be compared to those from a matched control group of drivers.

A number of driving simulator studies have shown that marijuana does not produce the kind of psycho-motor impairment evident with modest doses of alcohol. 80 In fact, in a recent NHTSA study, the only statistically significant outcome associated with marijuana was that drivers drove more slowly. 81

A recent study of actual driving ability under the influence of cannabis - employing the same protocol used to test the impairment-potential of medicinal drugs - evaluated the impact of placebo and three active THC doses in three driving trials, including one in high-density urban traffic.

    Dose-related impairment was observed in drivers' ability to maintain steady lateral position. However, even with the highest dose of THC, impairment was relatively minor - comparable to that with blood-alcohol concentrations of between .03 and .07 % and many legal medications. Drivers under the influence of marijuana also tended to decrease their speed and approach other cars more cautiously.

While recognizing some limitations of this study, the authors conclude that "THC is not a profoundly impairing drug." 82

~~Link to footnotes
~~Link to article

[Uzi578]

Marijuana Does Not Cause Reckless Driving

Source: DrugSense Weekly
September 26, 2003 Issue #319
Article by: Mitch Earleywine, Ph.D. 

The White House Office of National Drug Control Policy (ONDCP) and certain Wisconsin legislators have launched a new crusade against "drugged driving," with a heavy emphasis on marijuana.  This crusade is largely based on scientific misinformation, and it could lead to the enactment of bad laws.

ONDCP has several slick television commercials on the subject.  One shows dramatic auto accidents and two crash test dummies passing a joint while a serious voice says, "In a recent study, one in three reckless drivers tested positive for marijuana." Note the careful phrasing.  The idea is to make viewers think that marijuana caused the reckless driving, without really saying that it did.

Why would ONDCP be so coy? The answer lies in the actual data regarding marijuana's effects on driving,

I study the effects of drugs and teach classes in the science of illicit substances, so I know this field.  The plain fact is that marijuana does not cause reckless driving.  Large studies of accidents show that drivers who test positive for marijuana (and ONLY marijuana -- i.e., people who haven't also been drinking or taking other intoxicating drugs) cause fewer crashes than people who haven't had any drugs at all.

That's right, people "high" on marijuana cause fewer crashes than those who are completely sober.  The findings seemed impossible to explain.  It was a puzzle that made no sense.

A bright and talented researcher in the Netherlands named Robbe recently solved that puzzle.  He got experienced marijuana users stoned and had them drive around the streets of Holland.  But these guys were no dummies.  They drove slower, increased the distance between their cars and the cars in front of them, and never tried to pass other cars.  Folks who smoked a placebo (a non-intoxicating substance made to look and smell like marijuana) drove as they usually did.  Alcohol, alone or in combination with marijuana, wrecked driving completely.

Robbe's results helped explain the accident studies.  People who used marijuana and only marijuana were compensating for the drug's effects by driving more carefully.  Nobody should drive high, but we can all take a lesson from these people who did: slow down, leave space between your car and the next, and don't try to pass.  Unlike alcohol, which makes people behave recklessly, marijuana users tend to be aware that they are impaired and compensate with some success.

But what about the ONDCP's claim that one in three reckless drivers tested positive for marijuana?

It's not quite a lie, but it's deliberately misleading.  The Drug Czar's no dummy.  He wants to scare people, and he knows the complete facts won't do it.  Instead he throws out scary but incomplete and misleading statistics -- and hopes people won't question them.  Yes, one in three reckless drivers tested positive for marijuana in a urine screen, but we don't know how many of them had alcohol, antihistamines, cocaine, or any number of other drugs in their systems.

Legislators need to ask for the complete facts behind the scare stories before they start passing new laws based on misinformation.

There are cheaper, easier ways to get impaired drivers off the road.  Roadside sobriety tests are reliable, inexpensive, and valid indicators of impaired driving.  Law-enforcement officers can learn to administer these tests quickly and easily.  Unlike expensive blood tests, which can only identify a few drugs, roadside sobriety tests can detect any kind of drug impairment that might hurt driving.  People who've had too many antihistamines can't drive well.  Roadside sobriety tests would keep them off the road.  A blood test would let them drive on by.

Don't be a dummy.  Insist on roadside sobriety tests instead of expensive, misleading blood tests.

Mitch Earleywine, Ph.D., is an associate professor of psychology at the University of Southern California and author of "Understanding Marijuana" (Oxford University Press, 2002). 

~~Link to article

[Uzi578]

Marijuana and Actual Driving Performance Executive Summary

National Highway Traffic Safety Administration
By Robbe HWJ, O'Hanlon JF
November 1993

Conducted by Institute for Human Psychopharmacology University of Limburg Abstract 2A-6211 LS Maastricht -- Netherlands
Sponsoring Agency: U.S. Department of Transportation National Highway Traffic Safety Administration 400 Seventh Street, S.W. Washington, DC 20590

Disclaimer

This publication is distributed by the U.S. Department of Transportation, National Highway Traffic Safety Administration, in the interest of information exchange. The opinions, findings and conclusions expressed in this publication are those of the author(s) and not necessarily those of the Department of Transportation or the National Highway Traffic Safety Administration. The United States Government assumes no liability for its contents or use thereof. If trade or manufacturers' name or products are mentioned, it is because they are considered essential to the object of the publication and should not be construed as an endorsement. The United States Government does not endorse products or manufacturers.

Abstract

Abstract: This report concerns the effects of marijuana smoking on actual driving performance. It presents the results of one pilot and three actual driving studies. The pilot study's major purpose was to establish the THC dose current marijuana users smoke to achieve their desired "high". From these results it was decided that the maximum THC dose for subsequent driving studies would be 300 mcg / kg (0.3 mg / kg). The first driving study was conducted on a closed section of a primary highway. After smoking marijuana delivering THC doses of 0, 100, 200, and 300 mcg / kg, subjects drove a car while maintaining a constant speed and lateral position. This study was replicated with a new group of subjects, but now in the presence of other traffic. In addition, a car following test was executed. The third driving study compared the effects of a modest dose of THC (100 mcg / kg) and alcohol )BAC of 0.04 g %) on city driving performance. This program of research has shown that marijuana, when taken alone, produces a moderate degree of driving impairment which is related to the consumed THC dose. The impairment manifests itself mainly in the ability to maintain a steady lateral position on the road, but its magnitude is not exceptional in comparison with changes produced by many medicinal drugs and alcohol. Drivers under the influence of marijuana retain insight in their performance and will compensate where they can, for example, by slowing down or increasing effort. As a consequence, THC's adverse effects on driving performance appear relatively small.

Executive Summary

This report concerns the effects of marijuana smoking on actual driving performance. It presents the results of one pilot and three actual driving studies which were conducted between April 1990 and March 1992. The program was funded by the U.S. National Highway Traffic Safety Administration (NHTSA), with the exception of the alcohol part of the city driving study which was sponsored by the Dutch Ministry of Transport and Public Works. The project was conducted by the Institute for Drugs, Safety and Behavior of the University of Limburg, Maastricht, The Netherlands. The major objectives of the program were to determine the dose-response relationship between delta-9-tetrahydrocannabinol (THC), marijuana's main constituent, and objectively and subjectively measured aspects of real-world driving; and, to determine whether it is possible to correlate driving performance impairment with plasma concentrations of the drug or a metabolite. A variety of driving tests were employed, including: maintenance of a constant speed and lateral position during uninterrupted highway travel, following a leading car with varying speed on a highway, and city driving. The purpose of applying different tests was to determine whether similar changes in performance under the influence of THC occur in all, thereby indicating a general drug effect on driving ability.

Chapter One provides background information about the drug, its pharmacological properties, the prevalence of its use, and a review of marijuana smoking and traffic safety. THC's effects on the ability of drivers to operate safely in traffic situations have traditionally been determined in two ways: from epidemiological surveys of users' involvement in traffic accidents and from empirical studies to measure the drug's influence on skills related to driving, or driving itself. Epidemiology shows that people drive after marijuana use and that drivers involved in accidents often show the drug's presence. The results are, however, inconclusive because of the high proportion of cases which also involve alcohol use and the lack of proper control groups. There, the extent marijuana contributes to traffic accident causality remains obscure. Results from driving simulator and closed-course tests show that THC in single inhaled doses up to about 250 mcg / kg has relatively minor effects on driving performance, certainly less than blood alcohol concentrations (BAC's) in the range of 0.08-0.10 g-%.

Chapter Two describes the studies of the program and certain procedures that were common to all. These were subject recruiting, compliance with ethical and legal standards, screening for the presence of other illicit drugs and alcohol, blood sampling procedures and quantitative analyses. Subjects in all studies were recreational users of cannabis, i.e. smoking marijuana or hashish more than once a month but not daily. They were all healthy, between 21 and 40 years of age, had normal weight and binocular acuity, and were licensed to drive an automobile. Subjects were accompanied in every driving test by a licensed driving instructor, experienced in supervising subjects who operated under the influence of medicinal drugs in previous studies. Redundant control system in the test vehicle was available for controlling the car if emergency situations should arise. Marijuana and placebo marijuana cigarettes were supplied by the U.S. National Institute on Drug Abuse (NIDA). [Isn't it curious that a study done in the Netherlands used NIDA supplied marijuana? It is nearly IMPOSSIBLE to get NIDA supplied marijuana in the U.S. for these kind of studies or medical studies.]

Chapter Three presents the results of the pilot study. It was conducted in a hospital under strict medical supervision to identify THC doses that recreational marijuana users were likely to consume before driving. Twenty-four subjects, twelve males and twelve females, participated. They were allowed to smoke part or all of the THC content in three cigarettes until achieving the desired psychological effect. Cigarettes were smoked through a plastic holder in a manner determined by the subjects. The only requirement was to smoke continuously for a perod not exceeding 15 minutes. When subjects voluntarily stopped smoking, cigarettes were carefully extinguished and retained for subsequent gravimetric estimation of THC consumed. Six subjects consumed one cigarette, thirteen smoked two and four smoked three. The average amount of THC consumed was 20.8 mg, after adjustment for body weight, 308 mcg / kg [of body weight]. There was no significant difference between males and females with respect to the weight adjusted preferred dose. It was decided that the maximum dose for subsequent driving studies would be 300 mcg / kg. This is considerably higher than doses that have usually been administered to subjects in experimental studies (typically, 100-200 mcg / kg THC).

The study provided the opportunity for obtaining valuable information about THC's pharmacokinetics and its pharmacodynamic effects after marijuana smoking. Blood samples were repeatedly taken for measuring plasma concentrations of THC and its major inactive metabolite, THC-COOH. The subjects repeatedly performed certain simple laboratory tests, estimated their levels of intoxication and indicated their willingnes to drive under several specified conditions of urgency. Heart rate was measured at these times. The secondary purpose of the pilot study was that of specifying relationships between (THC) and (THC-COOH) with changes in the other physiological, performance or subjective variables. Other results from the pilot study showed that perceived "high" and heart rate are very sensitive measures of marijuana intoxication which confirms prior findings. Impairments in laboratory tests performance were found at the time of peak subjective feelings but generally, objective impairment dissipated more rapidly than lthe feelings themselves.

The first driving study, described in Chapter Four, was conducted on a highway closed to other traffic. One objective of the study was to determine whether it would be safe to repeat the study on a normal highway in the presence of other traffic. The second objective was to define the dose-effect relationship between inhaled THC dose and driving performance. The same twelve men and twelve women who participated in the pilot study served again as the subjects. They were treated on separate occasions with THC doses of 0, 100, 200, 300 mcg / kg. Treatments were administered double-blind and in a counterbalanced order. On each occasion, subjects performed a road tracking test beginning 40 minutes after initiation of smoking and repeated one hour later. The test, developed and standardized by O'Hanlon et al. (1982, 1986), involved maintaining a constant speed at 90 km / h (56 mph) and a steady lateral position between the delineated boundaries of the traffic lane. Subjects drove 22 km (13.6 mi) on a primary highway and were accompanied by a licensed driving instructor. The latter was charged with responsibility for ensuring safety at all times and was able to intervene, if necessary, using redundant vehicular controls. The primary dependent variable was the standard deviation of lateral position (SDLP), which has been shown to be both highly reliable and very sensitive to the influence of sedative drugs and alcohol. Other dependent variables were mean speed, and standard deviation of speed and steering wheel angle. Blood samples were taken prior to each driving test; and, performance in critical tracking and hand steadiness tests, heart rate, and blood pressure were measured after its termination. Questionnaires were repeatedly administered to estimate the "high" and other subjective feelings.

All subjects were willing and able to finish the driving tests without great difficulty. The study demonstrated that marijuana impairs driving performance as measured by an increase in SDLP; all three THC doses significantly affected SDLP relative to placebo. The driving performance decrement after smoking marijuana persisted almost undiminished for two hours after smoking while drug plasma concentrations, perceived "high" and heart rate elevation had decreased. Marijuana's effects on SDLP were compared to those of alcohol obtained in a very similar study by Louwerens et al. (1985, 1987). It appeared that THC's effects on SDLP were equivalent to those associated with BAC's in the range of 0.03-0.07 g %. Other driving performance measures were not significantly affected by THC. Intersubject correlations between plasma concentrations of the drug and driving performance after every dose were essentially nil. Thus, driving impairment cannot be predicted by prevailing plasma concentrations of THC or THC-COOH. Driving impairment was also not related to performance in the laboratory tests. Both the observed degree of driving impairment, and what subjects said and did, indicated that normal safeguards would be sufficient for ensuring safety in further testing. Hence, the final conclusion was to repeat this study on a normal highway in the presence of other traffic.

The second driving study, described in Chapter Five, was conducted to come a step closer to driving reality than its predecessor. Driving tests were now conducted on a highway in the presence of other traffic. The major objective of this study was to confirm the relationship between inhaled THC dose and lateral position variability in the context of a standard road tracking test. A secondary objective was to measure performance in another actual driving test, i.e. car following. The third objective was to continue efforts to correlate plasma concentrations of THC and THC-COOH with driving performance impairment as measured in both tests.

A new group of sixteen subjects, equally comprised of men and women, participated in this study. A conservative approach was chosen in designing the present study in order to satisfy the series design where both active drug and placebo conditions were administered, double-blind, at each of three THC dose levels. THC doses were the same as those used in the previous study, namely 100, 200, and 300 mcg / kg. Cigarettes appeared identical at each level of treatment conditions and were smoked through a plastic holder in a fashion determined by the subject within a time limit of 10 minutes. If any subject would have reacted in an unacceptable manner to a lower dose, he / she would not have been permitted to receive a higher dose.

Two subjects at a time commenced smoking. Thirty minutes after onset of smoking the subjects performed a battery of laboratory tests (tracking, hand steadiness and body sway), yielded a blood sample, and rated their "high" and other subjective feelings. They were then transported to a primary highway where the driving tests were performed. Two instrumented vehicles were employed. The subjects performed the car following test on a 16 km (9.9 mi) segment of the highway for about twelve minutes. After conclusion of the car following test, both subjects then commenced the road tracking test in separate instrumented vehicles. The highway was the same as for the car following test. Subjects drove 64 km (40 mi) without stopping in about 50 minutes. At the conclusion of this test, both subjects participated again in the car following test. Subjects were then transported back to the laboratory where they rated subjective feelings, yielded a blood sample, and repeated the test battery. The subjects' heart rate was registered continuously during both driving tests.

The road tracking test was the same as in the previous study except for its duration and the presence of other traffic. Subjects were instructed to maintain a constant speed of 95 km / h (59 mph) and a steady lateral position between lane boundaries in the right traffic lane. They were allowed to deviate from this only if it would become necessary to pass a slower vehicle in the same lane. Data from the standart test were analyzed to yield the same performance measures as in the previous study; i.e. SDLP, mean and standard deviation of speed, and standard deviation of steering wheel angle. The car following test measures drivers' ability to perceive changes in a preceding vehicle's speed and to react in a manner maintaining a constant headway. It began as the preceding and the following vehicle, respectively driven by one of the driving instructors and the subject, operated in tandem on the slower traffic lane while travelling at a speed of 100 km / h (62 mph). The subject was instructed to maintain a 50 m (164 ft) headway however the preceding vehicle's speed might vary. After driving in this manner for about one minute, the operator of the preceding vehicle released the accelerator pedal allowing its speed to fall to 80 km / h (50 mph). Immediately thereafter, the operator of the preceding vehicle accelerated to 100 km / (62 mph). The duration of one deceleration and acceleration maneuver was approximately 50 seconds and six to eight, depending upon traffic density, were executed during one test. The subject's average reaction time to the movements of the leading vehicle, mean headway and coefficient of variation of headway during maneuvers were taken as the dependent variables from this.

All subjects were able to complete the series without suffering any untoward reaction while driving. Road tracking performance in the standard test was impaired in a dose-related manner by THC and confirmed the results obtained in the previous closed highway study. The 100 mcg / kg dose produced a slight elevation in mean SDLP, albeit nearly significant. The 200 mcg / kg dose produced a significant elevation, of dubious practical relevance. The 300 mcg / kg dose produced a highly significant elevation which may be viewed as practically relevant but unexceptional in comparison with similarly measured effects of many medicinal drugs. Following marijuana smoking subjects drove with an average speed that was only slightly lower than after placebo and very close to the prescribed level.

In the car following test, subjects maintained a headway of 45-50 m (148-164 ft) while driving in the successive placebo conditions. They lengthened mean headway by 8, 6 and 2 m (26.2, 19.7 and 6.6 ft) in the corresponding THC conditions after 100, 200 and 300 mcg / kg, respectively. The initially large drug-placebo difference and its subsequent decline is a surprising result. Our explanation for this observation is that the subjects' caution was greatest the first time they undertook the test under the influence of THC and progressively less thereafter. Reaction time to changes in the preceding vehicle's speed increased following THC treatment, relative to placebo. The administered THC dose was inversely related to the change in reaction time, as it was to headway. However, increased reaction times were partly due to longer headway. Statistical adjustment for this confounding resulted in smaller and non-significant increases in reaction time following marijuana treatment, the greatest impairment (0.32 s) being observed in the first test following the lowest THC dose. Headway variability followed a similar pattern as mean headway and reaction time; the greatest impairment was found following the lowest dose.

An important practical objective of this study was to determine whether degrees of driving impairment can be accurately predicted from either measured concentrations of THC in plasma or performance measured in potential roadside "sobriety" tests of tracking ability or hand and posture stability. The results, like many reported before, indicate that none of these measures accurately predicts changes in actual driving performance under the influence of THC.

The program then proceeded into the third driving study, presented in Chapter Six, which involved tests conducted in high-density urban traffic. There were logical and safety reasons for restricting the THC dose to 100 mcg / kg. It was given to a group of regular cannabis users, along with placebo. For comparative purposes another group of regular alcohol users were treated with a modest dose of their preferred recreational drug, and again placebo, before undertaking the same city driving test. Two groups of sixteen new subjects apiece, equally comprised of men and women, participated. Subjects in the alcohol group were regular users of alcohol but not marijuana. Both groups were treated on separate occasions with active drug and placebo. Active marijuana was administered to deliver 100 mcg / kg THC. The driving test commenced 30 minutes after smoking. The alcohol dose was chosen to yield a BAC approaching 0.05 g % when the driving test commenced 45 minutes after onset of drinking. Active drug and placebo conditions were administered double-blind and in a counterbalanced order in each group.

Driving tests were conducted in daylight over a constant 17.5 km (10.9 mi) route within the city limits of Maastricht. Subjects drove their placebo and active drug rides through heavy, medium and low density traffic on the same day of the week, and at the same time of day. Two scoring methods were employed in the present study. The first, "molar" approach, required the driving instructor acting as the safety controller during the tests to retrospectively rate the driver's performance using a standard scale. The second, a more "molecular" approach, involved the employment of a specially trained observer who applied simple and strict criteria for recording when the driver made or failed to make each in a series of observable responses at predetermined points along a chosen route. Immediately prior to and following the driving tests subjects performed hand steadiness and time perception tests, yielded a blood sample, and were administered the same subjective questionnaires used in the previous studies.

The study showed that a modest dose of alcohol (BAC = 0.04 g %) produced a significant impairment in city driving as measured by the molar approach, relative to placebo. More specifically, alcohol impaired vehicle handling and traffic maneuvers. Marijuana, administered in a dose of 100 mcg / kg THC, on the other hand, did not signifcantly change mean driving performance as measured by this approach. Neither alcohol nor marijuana significantly affected driving performance measures obtained by the molecular approach indicating that it may be relatively insensitive to drug-induced changes.

Driving quality as rated by the subjects contrasted with observer ratings. Alcohol impaired driving performance according to the driving instructor but subjects did not perceive it; marijuana did not impair driving performance but the subjects themselves perceived their driving performance as such. Both groups reported about the same amount of effort in accomplishing the driving test following placebo Yet only subjects in the ;marijuana group reported significantly higher levels of invested effort following the active drug. Thus, there was evidence that subjects in the marijuana group were not only aware of their intoxicated condition but were also attempting to compensate for it. These seem to be important findings. They support both the common belief that drivers become overconfident after drinking alcohol and investigators' suspicions that they become more cautious and self-critical after consuming low THC doses by smoking marijuana.

The laboratory performance tests also discriminated between the drugs' effects. Hand steadiness was impaired following THC and improved following alcohol, relative to placebo. The difference between the drugs' effects was significant, both before and after the driving test. Impairment after THC was about as much as that produced by the same dose in the previous study, indicating equivalent sensitivities of the present and previous groups. Production of time intervals was not affected by alcohol, but THC significantly shortened interval production, relative to placebo.

Drug plasma concentrations were neither related to absolute driving performance scores nor to the changes that occurred from placebo to drug conditions. With respect to THC, these results confirm the findings in previous studies. They are somewhat surprising for alcohol but may be due to the restricted range of ethanol concentrations in the plasma of different subjects.

Chapter Seven concludes the report with a general discussion of the results of the program and ends with list of conclusions and recommendations. It starts with a discussion of the THC dose which marijuana users actually prefer for achieving their desired "high." Several questions are raised and discussed, such as: how do people regulate their THC consumption, what role plays familiarization with the drug, and what would the preferred dose have been if marijuana of much higher potency were smoked. The discussion then continues with a description of the differences between the driving tests in terms of the type of information processing each requires, automatic vs controlled, and the relevance of each to traffic safety.

Attention is further focussed on the effects of THC on driving performance. The results of the studies corroborate those of previous driving simulator and closed-course tests by indicating that THC in single inhaled doses up to 300 mcg /kg has significant, yet not dramatic, dose-related impairing effects on driving performance. Standard deviation of lateral position in the road tracking test was the most sensitive measure for revealing THC's adverse effects. This is because road tracking is primarily controlled by an automatic information processing system which operates outside of conscious control The process is relatively impervious to environmental changes but highly vulnerable to internal factors that retard the flow if information through the system. THC and many other drugs are among these factors. When they interfere with the process that restricts SDLP, there is little the afflicted individual can do by way of compensation to restore the situation. Car following and, to a greater extent, city driving performance depend more on controlled information processing and are therefore more accesible for compensatory mechanisms that reduce the decrements or abolish them entirely.

It appears that performance is more affected by THC in laboratory than actual driving tests. Several reasons that may account for the apparent discrepancy are discussed. First, laboratory tests are experimentally controlled by drastic simplification which may affect a subjects motivation to perform the test by making it appear "unreal." Secondly, the restriction of response options in laboratory performance tests leave fewer possibilities for compensation. In real life, drivers always apply numerous skills in parallel and series. Should one become deficient, they are often able to compensate in a number of ways to achieve a satisfactory level of proficiency. Finally, after learning to drive, subjects possess such skills in abundance and one can only demonstrate how they vary with drug effects in the real task or a very close approximation thereof. Profound drug impairment constituting an obvious traffic safety hazard could as easily be demonstrated in a laboratory performance test as anywhere else. But THC is not a profoundly impairing drug. It does affect automatic information processing, even after low doses, but not to any great extent after high doses. It apparently affects controlled information processing in a variety of laboratory tests, but not to the extent which is beyond the individual's ability to control when he is motivated and permitted to do so in real driving.

Marijuana's effects on driving performance were compared to those of many other drugs. It was concluded that THC's effects after doses up to 300 mcg / kg never exceed alcohol's at BAC's of 0.08 g %; and were in no way unusual compared to many medicinal drugs'. Yet THC's effects differ qualitatively from many other drugs, especially alcohol. Evidence from the present and previous studies stronly suggests that alcohol encourages risky driving whereas THC encourages greater caution, at least in experiments. Another way THC seems to differ qualitatively from many other drugs is that the former's users seem better able to compensate for its adverse effects while driving under the influence. Still one can easily imagine situations where the influence of marijuana smoking might have an exceedingly dangerous effect; i.e., emergency situations which put high demands on the driver's information processing capacity, prolonged monotonous driving, and after THC has been taken with other drugs, especially alcohol

Finally, the relation between driving impairment following marijuana smoking and plasma concentrations of THC and THC-COOH is discussed. It appears not possible to conclude anything about a driver's impairment on the basis of his / her plasma concentrations of THC and THC-COOH determined in a single sample.

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[Uzi578]

Part 1 of 4

Marijuana, Alcohol and Actual Driving Performance

By Hindrik W. J. Robbe, Ph.D. and James F. O'Hanlon, Ph.D.
1999?

CONTENTS

    * Document Header
    * Acknowledgements
    * Executive Summary
    * 1. INTRODUCTION
    * 2. METHODS
    * 2.1 Subjects
    * 2.2 Legality and Ethics
    * 2.3 Design, Doses and Administration
    * 2.4 Procedures
    * 2.5 Driving Tests and Rating Scales
    * 2.6 Statistical Analysis
    * 3. RESULTS
    * 3.1 Adjustments for Missing Data
    * 3.2 Blood Alcohol Concentration (BAC)
    * 3.3 Road Tracking
    * 3.4 Car Following
    * 3.5 Intoxication Ratings
    * 3.6 Driving Quality Ratings
    * 3.7 Instructor's Comments
    * 4. DISCUSSION
    * 5. CONCLUSIONS
    * 5.1 General Conclusions
    * 5.2 Specific Conclusions
    * 6. REFERENCES

Document Header

[Erowid Note: Although this document has a year of 1999 on it, it is written by the same authors (Robbe & O'Hanlon) and appears to be based on the same research as the earlier work, from 1993. The executive summary written in 1993 can be found here.]

U.S. Department of Transportation
National Traffic Safety Administration
DOT HS 808 939
Marijuana, Alcohol and Actual Driving Performance
July 1999
This document is available to the public from the National Technical Information Service, Springfield, Virginia 22161.
This publication is distributed by the U.S. Department of Transportation, National Highway Traffic Safety Administration, in the interest of information exchange. The opinions, findings and conclusions expressed in this publication are those of the author(s) and not necessarily those of the Department of Transportation or the National Highway Traffic Safety Administration. The United States Government assumes no liability for its contents or use thereof. If trade or manufacturer's name or products are mentioned, it is because they are considered essential to the object of the publication and should not be construed as an endorsement. The United States Government does not endorse products or manufacturers.

Technical Report Documentation
1. Report No. 1
2. Government Accession No. DOT HS808 939
3. Recipient's Catalog No.
4. Title and Subtitle
5. Report Date
6. Perforrning Organization Code IHP
7. Author(s) Hindrik W. J. Robbe, Ph.D. and James F. O'Hanlon, Ph.D.
8. Performing Organization Report No. P44 E1
9. Performing Organization Name and Address Institute for Human Psychophamacology Maastricht University P. O. Box 616 6200 MD Maastricht, The Netherlands
10. Work Unit No. (TRAIS)
11. Contract or Grant No.
12. Sponsoring Agency Narne and Address
13. Type of Report and Period Covered
14. Sponsoring Agency Code
15. Supplementary Notes 6 abstract
16. Purpose: The purpose of this study was to empirically determine the separate and combined effects of Delta 9 tetrahydrocannabinol (THC) and alcohol on actual driving performance. This was the first study ever in which the drugs' combined effects were measured in a natural setting, i.e., on real roads in normal traffic.

Eighteen subjects between 20 and 28 years of age who were licensed to drive an automobile and who smoked marijuana and drank alcohol at least once per month participated in the study. They were treated with drugs and placebo according to a balanced,way, observer and subject blind, cross over design. On separate evenings they were given weight calibrated doses of THC and alcohol, or placebos for one or both substances as follows: alcohol placebo + THC placebo; alcohol placebo + THC 100 :g/kg; alcohol placebo + THC 200 :g/kg; alcohol + THC placebo; alcohol + THC 100 :g/kg; and, alcohol + THC 200 :g/kg. The initial alcohol dose was sufficient for achieving a peak blood alcohol concentration (BAC) of about 0.07 g/dl. Booster doses were later given to sustain BAC around 0.04 g/dl during testing.

At each occasion, 30 min. after smoking, subjects performed four driving tests in the evening hours after dark between 21:00 and 23:15: two repetitions of a Road Tracking Test and two repetitions of a Car Following Test. The former test measured the subjects' ability to maintain a constant speed of 100 km/in (62 mph) and a steady lateral position between the delineated boundaries of the right (slower) traffic lane. The latter test measured the subjects' reaction times and headway variability while driving 50 m (164 ft) behind a preceding vehicle that executed a series of alternating acceleration and deceleration maneuvers. Both THC doses alone, and alcohol alone, significantly impaired the subjects' Road Tracking and Car Following performances. The magnitude of the mean effects were minor after alcohol and THC 100 :8/kg and moderate after THC 200 :g/kg. Both THC doses in combination with alcohol severely impaired the subjects' performance in both tests. While the effects of THC alone in doses up to 200 :8/kg might be categorized as "moderate", they become "severe" when THC is combined with a moderate dose of alcohol.

17. Key Words Alcohol, marijuana, driving performance, ethanol, THC,highway safety, traffic safety, drugs and driving
18. Distribution Statement: Document available to the public through the National Technical Information Service,Springfield, VA 22161
19. Security Classif (of this repot) : Unclassified
20. Security Classif (of this page) : Unclassified
21. No of Pages: 22

Form DOT F 1700.7 (8 72) Reproduction of completed page authorized This form was electronically produced by Elite Federal Forms, Inc. Marijuana/Alcohol Driving Study

ACKNOWLEDGMENTS

This report is the second that emerged from programmatic marijuana research conducted by the Institute for Human Psychopharmacology, Maastricht University, under contract to the National Highway Traffic Safety Administration (NHTSA) of the US Department of Transportation. We thank NHTSA for supporting the research program and Mr. Ted Anderson and Dr. James Frank for monitoring it. We appreciate the cooperation of the US National Institute on Drug Abuse (NIDA) for providing the marijuana cigarettes.

The contract has been locally administered by the Dutch Road Safety Directorate of the Ministry of Transport and Public Works under an intergovernmental agreement, and we would like to thank Dr. Jan Busstra for monitoring the project locally.

We are grateful to the District Attorney, Mr. H.W. Overbosch, and the Maastricht University's Institutional Review Board for reviewing and approving the study. A permit for obtaining, storing and administering marijuana was obtained from the Dutch Drug Enforcement Administration.

We'd like to thank our colleagues, Cees van Leeuwen, MD, and Nienke Muntjewerff, MD, for the medical supervision; Anita van Oers and Aldrik Schmitz du Moulin for their assistance during data collection; and, Henk Brauers and Pitou Sweens, our licensed driving instructors, for their important contribution of supervising the subjects while driving. Finally we thank the subjects for their participation.

EXECUTIVE SUMMARY

Previous driving simulator and closed.course studies have failed to confi rm what epidemiologists' data suggest, i.e., that alcohol and Delta-9-tetrahydrocannabinol (THC) interact synergistically to produce greater impairment than the sum of the changes that each drug produces separately. Yet with the methodological shortcomings of particular studies and the general lack of test realism afflicting all, it can not be said that their failure to measure a synergistic effect is particularly convincing.

The study described in this report represents another attempt to empirically determine the separate and combined effects of THC and alcohol on driving. It differs from its predecessors by employing standardized tests for objectively measuring the drugs' effects on driving performance in the natural environment; i.e., on real roads in normal traffic..

Eighteen volunteer subjects, comprised of men and women in equal proportions, participated in the present study. The subjects were all university students between 20 and 28 years of age who smoked marijuana and drank alcohol at least once per month. All admitted having previously driven under the influence of each drug separately but only three, of both together. They were treated with drugs and placebo according to a balanced, 6.way, observer. and subject.blind, cross.over design. On separate evenings they were given weight.calibrated doses of THC and alcohol, or placebos for one or both substances as follows: alcohol placebo + THC placebo; alcohol placebo + THC 100 ~g/kg; alcohol placebo + THC 200 1lg/kg; alcohol + THC placebo; alcohol +THC 100 ~g/kg; and, alcohol +THC 200 1lg/kg. The initial alcohol dose was sufficient for achieving a peak blood alcohol concentration (BAC) of about 0.07 g/dl. Booster doses were later given to sustain BAC around 0.04 g/dl during testing. THC was administered by smoking marijuana cigarettes (2.2 and 3.95% THC from the US National Institute on Drug Abuse, NIDA) which had been weight.calibrated by cutting. Initial drinking preceded smoking by 60 min.

Driving tests began 30 min. after smoking at 21:00 hr. Subjects undertook them in pairs on the same evening. One started with the Car Following Test and the other 4 min. Iater with the Road Tracking Test. After driving on the highway for approximately 25 min., the first subject drove off and awaited the second. When he/she arrived, the pair exchanged roles, returned to the highway, and drove in the reverse direction until returning to the origin. The same procedure was repeated twice so that each subject completed two repetitions of each test. Testing concluded at approximately 23:15 hr.

Subjects were accompanied by licensed driving instructors having access to redundant vehicle controls. In the Road Tracking Test, the subject attempted to maintain a constant speed of 100 km/in (62 mph) and a steady lateral position between the delineated boundaries of the right (slower) traffic lane. Standard deviation of lateral position (SDLP) was the primary outcome variable. SDLP is a measure of road tracking error, in practical terms, a composite index of allowed weaving, swerving and overcorrecting. Failures to restrict the vehicle's lateral motion within lane boundaries were recorded together as the percentage of time out of lane (TOL). The Car Following Test involved the use of two vehicles. The preceding vehicle was under an investigator's control, and the following vehicle, the subject's. The test began with the two vehicles traveling in tandem at speeds of 100 km/in (62 mph). Subjects attempted to drive 50 m (164 ft) behind the preceding vehicle and to maintain that headway as it executed a series of alternating acceleration and deceleration maneuvers lasting 33 sec apiece. The investigator driving the preceding vehicle initiated each maneuver by activating a microprocessor.driven cruise.control. The vehicle's speed then rose or fell in a constant manner until arriving at a point 15 km/in (9.3 mph) higher or lower than where it began. The investigator drove at the newly established speed for 0.5.5.0 min. before initiating the next maneuver. About eight maneuvers in each direction were accomplished over both repetitions of the test. Headway was continuously recorded as were the subject's discrete reaction times (RTs) at the beginning of maneuvers. Average RT and the standard deviation of headway (HSD) for acceleration and deceleration maneuvers, separately, were the major dependent variables. Subjects rated their degrees of intoxication before and after the entire series of tests. They also rated the quality of their driving performance at the end of each test repetition. The instructors likewise rated the subject's driving quality at the same times and in the same manner.

In statistical terms, both THC doses alone, and alcohol alone, significantly impaired the subjects' Road Tracking and Car Following performances. In practical terms, the magnitude of the mean effects were minor after alcohol and THC 100 ~g/kg and moderate after THC 200 ~g/kg. Both THC doses in combination with alcohol severely impaired the subjects performance in both tests. The mean changes in SDLP from the placebo level after combinations involving THC 100 and 200 ~g/kg were evaluated relative to a previously established alcohol calibration curve (Louwerens et al, 1987). The former combination produced a rise in mean SDLP the equivalent of that associated with BAC=0.09 g/dl, and the latter, the equivalent of BAC=0.14 g/dl. Mean TOL rose exponentially with SDLP. Beginning at the placebo level of 0.2%, mean TOL increased with the severity of drug effects until reaching 1.1% after the combination of alcohol and THC 200 1lg/kg. Mean RT and HSD during deceleration maneuvers varied across treatment conditions in the same manner. Beginning at the placebo level of 4.65 see, mean RT lengthened to 6.33 sec (+36%) under the combined influence of alcohol and THC 200 ug/kg. The change in mean HSD was from 5.69 to 7.78 m (+37%). The subjects' and instructors' rating of driving quality clearly reflected the adverse objective effects of alcohol and THC alone and in combination. In addition, the instructors spontaneously recorded several cases, usually in combined drug conditions, wherein a subject's aberrant behavior would have been dangerous were he/she to operate the same way under natural driving conditions.

We concluded that marijuana smoking that delivers the relatively low.moderate THC doses of 100 and 200 ~g/kg impairs Road Tracking and Car Following performance. THC effects are dose.related and persist unabated or even increase during 2 1/2 be after dosing. The magnitudes of impairment observed after these doses of THC alone were not especially large in historical comparison to those of other drugs and alcohol present in BACs above 0.08 g/dl. However, they do imply a loss of driving ability that could be serious in exceptional traffic situations. The combination of THC with alcohol sufficient for attaining a BAC of about 0.04 g/dl has very severe effects on driving performance. Subjects showing those effects drove in a manner one would expect for drivers operating with BACs above the per se definition of intoxication in some states, i.e., 0.08 g/dl. No unequivocal evidence emerged from this study to indicate that the drug interaction is synergistic in the classic pharmacological sense. However, the exponential rise of TOL from conditions where THC and alcohol were given separately to those where they acted in combination suggests that the interaction can increase the risk of certain types of crashes in the same manner. That being the case, the practical consequences of driving after the combined use of THC and alcohol would be the same whether their pharmacological interaction is synergistic or merely additive.

~~Link to article

[Uzi578]

Part 2 of 4

INRODUCTION

There is no doubt that D9-tetrahydrocannabinol (THC) impairs its users' cognitive and psychomotor abilities to an extent largely determined by the inhaled or ingested dose. It is also certain that the dose preferred by cannabis smokers (around 300 ~g/kg) is sufficient for impairing performance in potentially dangerous tasks such as driving (Robbe & O'Hanlon, 1993). It is less certain that those doses cause the degrees of impairment that seriously compromise driving ability, and if they do, whether many THC users choose to drive in that condition. Both the severity of the users' impairment and their prevalence among the driving population could be determined from an epidemiological survey measuring the relative frequencies among drivers who do and do not become involved in crashes. These data would permit calculation of the THC users' risk of crashing relative to that of drug.free drivers. Relative risk (RR) is in fact the only commonly accepted index of any drug's hazard potential for individual drivers, and in direct proportion to its usage prevalence, for the driving population as a whole. Yet more than 20 years of epidemiological research on THC has failed to establish the RR of drivers operating under its influence. Other results to emerge from numerous studies over this period are little more than suggestive. The epidemiologists' common failure is understandable. It is commonly thought that the prevalence of THC use in the normal population can only be determined from the drug's detection in blood samples provided by drivers at roadside checkpoints. With no means to compel or convince normal motorists to provide control data, epidemiologists have had to confine their attention to drivers from whom blood samples can be obtained after their admittance to hospitals as the result of injuries sustained in crashes. Thus, epidemiologists have only been able to measure the prevalence of THC use among injured drivers. Some epidemiologists have found a relatively high prevalence of THC in a limited geographical area, and others, significantly more THC users among drivers deemed responsible for crashes than apparently innocent victims. However, these results are equivocal. A high prevalence among injured drivers in one particular area might simply reflect a high usage prevalence among the local population. Methods for assigning responsibility have differed widely and those relying solely on police judgments may reflect their biases; e.g., suspected drug users may more often be judged responsible than is actually the case. Moreover, epidemiologists have always had to contend with a serious confounding factor; i.e., the simultaneous presence of alcohol in the majority of drivers who test positive for THC.

Surveys conducted in widely separated localities have generally revealed the presence of THC in between 4 and 12% of drivers who sustained injury or death in crashes (Cimbura et al, 1980,1982; Terhune, 1982; Chesher & Starmer, 1983; Donelson et al, 1985; Garriott et al, 1986; Daldrup et al, 1987; McClean et al, 1987). Occasionally, higher values have been reported for groups of, predominately, young males operating in one or another large American city (Williams et al, 1985; Soderstrom et al, 1988; Budd et al, 1989). Although the prevalence of THC users in the general driving population was assumed to be lower in almost every case, these data can not be accepted as evidence showing that THC was responsible for the crashes. The reason is that alcohol, usually in concentrations associated with a high crash risk, was also found in at least 50%, and sometimes as many as 90%, of the same drivers. The latest and largest of the postmortem surveys (Terhune et al, 1992) came closest to discriminating among the separate and combined effects of THC and alcohol on crash risk. It involved a sample of 1,882 fatally.injured drivers from seven widely separated American States during 1990.91. Drug.free drivers comprised 42.1% of the sample, and those showing the presence of alcohol, 51.5%. THC was found in only 4.3% and among them, three.quarters tested positive for alcohol as well. The investigators undertook two different analyses for inferring causal relationships between these drugs and crashes. First, they compared responsibility rates of subgroups using each drug separately and in combination with that for the drug.free group. Responsibility in this case was assigned by trained encoders using a standardized procedure for evaluating police reports. Drug.free drivers were held responsible for 67.7% of their crashes. The responsibility rates for drivers showing only the presence of alcohol depended upon their BACs. For those with BACs below 0.10 g/dl it was 75.8%, and for those at or above that level, 93.9%. Drivers showing only the presence of THC were 57.9% responsible; i.e., less often than drug.free drivers, albeit not significantly. However, the group showing the combined presence of THC and alcohol in any concentration at all, were held responsible for 94.6% of their crashes. This rate differed significantly from the drug.free drivers' though not from the group's with the highest BACs. The second analysis was for calculating the relative crash risk (RR) of each subgroup whose number was sufficient for providing a reliable estimate. "Non.responsible" drivers in the sample were defined as the control group, following the assumption that they were representative of the general driving population with respect to drug and alcohol prevalence. The investigators were aware that their definition of the control group is unconventional and urged caution in interpreting the results. But as they said, those results were certainly "suggestive." Relative to drug.free drivers whose M was defined as 1.0, those showing BAC<O. 10 g/dl operated with a RR=1.2. Drivers operating with BAC>= 0 10 g/dl did so with RR=6.5. Strikingly, drivers operating with THC and any BAC had a RR=11.9. Unfortunately, only 19 fatally injured drivers were found with only THC in their blood, too few for estimating the RR.

Terhune et al's results suggest a particularly dangerous synergistic (i.e., multiplicative) interaction between THC and alcohol but they carefully avoided drawing that conclusion. Their data were insufficient for confirming the impressions that drivers using only THC were less likely than drug.free drivers to be responsible for fatal crashes; and, that those showing the presence of both THC and alcohol were more likely to be responsible than others using alcohol alone. The data were also insufficient for determining whether the combined drug users' risk of fatal crash involvement increased more rapidly as a function of BAC than for the drivers using alcohol alone. Besides, their risk estimates are questionable since there is reason to doubt that "non.responsible" fatally injured drivers are representative of the driving population in general: some of them may have been responsible in the sense of failing to avoid the situations where the crashes occur. All doubts would have been avoided if the control group was comprised of drivers who were not involved in crashes. But this postmortem survey, like all before it, lacked the essential controls. Not surprisingly, the investigators concluded that it would be pointless to undertake even larger postmortem surveys in an effort to define THC's role in crash causality. Instead they recommended crash surveys involving the more numerous injured but surviving drivers whose accounts of antecedent events would better enable the assignment of responsibility; and, experimental studies designed specifically to show THC and alcohol effects, alone and in combination.

Numerous experimental studies have already been undertaken for that purpose (reviews: Chesher 1995; Robbe, 1994). Most are of limited relevance in the present context since the laboratory psychomotor tests they employed were short and relatively simple, bearing almost no resemblance to actual driving. It is only worth mentioning that no study measuring the separate and combined effects of up to three doses of both alcohol and THC has ever shown that the drugs' effects are more than additive. That is, their combined effects were essentially no greater than the sum of changes that each drug produced separately. Chesher noted, however, that the drugs' impairing effects in some laboratory tasks have differed qualitatively. Thus, the combination might simultaneously degrade different mental functions that independently affect performance in complex real.life tasks. That being the case, one might expect the simultaneous effects of THC and alcohol to seriously degrade complex performance, even if those effects are not synergistic in the classic pharmacological sense.

Driving is probably the most complex psychomotor task undertaken by ordinary individuals on a routine basis. It is difficult to conceive, much less simulate, every situation that confronts drivers. At best, tests for measuring drugs' effects in driving simulators, over a closed.course driving terrain or on real roads and in normal traffic, can measure only a few aspects of total driving behavior. Nonetheless, it would seem that the closer they approach reality, the better their likelihood of measuring the effects that cause crashes. This assertion has never been proven but its general acceptance is evident from the gradual development of more realistic tests, both for use in simulators and on the road.

Smiley et al (1981), conducted the first study of THC and alcohol effects in an interactive driving simulator. That system responded to the operator's control inputs by modifying its displayed visual imagery according to normal vehicle dynamics, though on a fixed base. The simulated tasks contained in a 45.mint scenario included driving on straight and curved road segments, following a vehicle and passing when gaps in the oncoming traffic permitted, changing the route in response to navigational information on signs and avoiding obstacles that suddenly appear on the roadway. A visual choice reaction time was also superimposed on driving. Three groups of marijuana users smoked cigarettes containing 0, 100 and 200 1lg/kg THC on two occasions per dose, once with and once without alcohol. The quantity of alcohol consumed varied between groups to reach blood concentrations of 0.00, 0.05 and 0.08 g/dl, respectively. To ensure high motivation, good driving was rewarded and blatant errors, such as crashes, were penalized financially. The test began 15 min. after the cessation of smoking. Both THC doses increased lateral position variability and the highest dose increased speed variability during curve following. Both THC doses increased headway variability, and the highest, lateral position variability during car following. Both caused the subjects to ignore navigational information. The high dose caused the subjects to hit roadway obstacles more often and to react more slowly in the subsidiary task than the placebo. Yet both THC doses caused the subjects to drive in a more conservative manner. They maintained a longer headway while car following, refused more opportunities to pass, and when they did, began this maneuver at a greater distance from the approaching vehicle. Alcohol's effects in this study were generally not significant. There were also no significant interactions between the drugs' effects on any performance measure.

Stein et al (1983) conducted two studies of alcohol and marijuana effects using a similar driving simulator and test scenario, lasting 15 min. Both studies followed a 2 (alcohol) x 3 (THC) cross.over design. Alcohol placebo and alcohol sufficient for producing a BAC of 0.10 g/dl were given in both. The THC doses were 0 (placebo), 50 and 100 ~g/kg in the first and 0, 100 and 200 ~g/kg in the second. As opposed to the lack of alcohol effects in the earlier simulator study, these two showed alcohol's expected, significantly adverse effects on practically every performance measure. Again in contrast to the earlier study, THC had almost no significant effects. The rather benign exception was that the subjects drove at a lower speed after the highest THC dose. The combination of drugs produced no consistent signs of a pharmacodynamic interaction affecting performance. It did, however, produce a marked rise in inter.subject performance variability, suggesting that some individuals might have been severely affected. The combination with the highest THC dose also produced significantly more "accidents" than alcohol did alone.

Four studies have followed the alternative approach of testing alcohol's and THC's effects on vehicle handling performances during staged maneuvers on a terrain closed to normal traffic (Casswell 1997; Attwood 1981; Smiley et al, 1986; Peck et al, 1986). Except for the last, these studies seem of minor importance. The driving tests varied from one to the other making their results difficult to compare. Casswell and Attwood treated small groups of male volunteers, 13 and 8, respectively, in cross.over designs with higher doses of each drug separately than when both were given in combination. THC doses were very low in all cases (<90 ~g/kg). Smiley et al administered more representative THC doses (100 and 200 ~g/kg) alone and in combination with alcohol sufficient for achieving a BAC of 0.05 g/dl, and also a higher dose of alcohol alone (BAC=0.08 g/dl), but to separate groups of 9 men. So the statistical power of their design was not greater than the others'. In any case the effects of THC and alcohol alone, and in combination, were modest in every study and no sign of a synergistic interaction was observed.

Peck et al's study was superior in most respects. It too followed a parallel group design with 21 male participants in each one. They were respectively treated with double placebo, alcohol (BAC=0.08 g/dl) + marijuana placebo, alcohol placebo + THC 19 ma, and both drugs combined. If these subjects could have consumed the entire THC dose, on the average it would have been about 270 ug/lkg. But considering the residual left after smoking, the actual average dose was probably closer to 250 ug/kg. The subjects were tested in four complete replications of test battery at 1.fur intervals beginning shortly after dosing. Ratings of their proficiency were obtained from accompanying driving licensing examiners, from observers stationed at points along the test route and from traffic police officers who followed the subjects' vehicle in a patrol car. A computerized system recorded the subjects' control movements and the vehicle's speed and lateral position relative to course delineation. A risk acceptance test was included for measuring the subjects' willingness and ability to drive through gaps that were slightly wider or narrower than the vehicle. Other tests involved stopping in response to signals, making a forced lane change and driving through a chicane. Several hundred performance variables were recorded and tested preliminary for selecting those that twice discriminated between treatment effects with a "significance" level of at least pobbe and O'Hanlon, 1993), as well as those of medicinal drugs in a series of more than 50 studies (reviews: O'Hanlon 1984; O'Hanlon e' al, 1986, 1995: O'Hanlon & Ramaekers 1995). It is hoped that the present study, and one more to follow, will conclusively demonstrate whether the drugs' combined effects are additive or synergistic, and how much in either case.

METHODS
Subject

Male and female volunteers were solicited by an advertisement in the newspaper of the Maastricht University. It described the general nature of the study and provided a telephone number to call for further information. Respondents were preliminarily screened to determine if they fulfilled the inclusion criteria: current use of both alcohol and marijuana with respective frequencies of once per week and once per month, but neither daily; possession of a valid driving license; driving experience of at least 1000 km/yr (620 mi/yr) over the previous three years; willingness to comply with certain restrictions of daily living activity (below); and, willingness to provide written informed consent. Those who apparently satisfied all criteria were sent Information for Volunteers, which completely described the study and its requirements, as well as questionnaires regarding personal medical history and experience with alcohol and drugs. Volunteers wishing to continue resumed contact for arranging a medical screening interview. The latter comprised a review of the completed questionnaires, a physical examination, a standard 12.lead ECG examination and the submission of blood and urine samples for routine clinical laboratory determinations. Fractions of the urine samples were retained and assayed qualitatively, on site, for drugs of abuse . amphetamines (including MDMA, called "ecstasy"), barbiturates, benzodiazepines, cannabinoids, cocaine and opioids. Urine from females was also assayed for beta.HCG to indicate pregnancy. Volunteers were excluded on the basis of results showing any of the following: history or evidence of drug or alcohol abuse or dependency; history of psychiatric or organic brain disorders; history or overt signs of serious cardiovascular, respiratory, renal, hepatic, metabolic or neuromuscular disorders; necessity for the chronic use of any systemic medication, except oral contraceptives; current use of any prescribed psychoactive medication; the presence of any drug of abuse in urine, besides cannabinoids; and for females, pregnancy or any reasonable possibility that pregnancy could occur during participation in the study. One unusual exclusion criterion was added by local law enforcement authorities as a condition for their approval of the study: volunteers having any record of arrests for drug trafficking were to be excluded. For this purpose the Chief District Attorney for the City of Maastricht reviewed a list of volunteers' names with their knowledge and consent. The District Attorney neither retained nor copied the list and he guaranteed no legal consequences for the individuals concerned. No volunteer was excluded by this procedure but common knowledge that it would be applied might have dissuaded some individuals from ever volunteering.

Eighteen subjects, 9 men and 9 women were selected. They ranged in age from 20 to 28 years (mean±SD, 22.7±2.1). All satisfied the inclusion criteria for driving experience with one exception; i.e., a woman had been licensed to drive for only 20 months before enrolling. She was allowed to enter the study for achieving a better balance between genders. The longest any subject had held a driving license before enrolling in the study was 9.5 yr. For the group as a whole, (mean±SD,, driving experience was 4.3±2.2 yr. The subjects declared their alcohol drinking frequency was from 1 to 30 glasses of wine or beer per week (14.2±8.8); and their marijuana smoking frequency from 1 to 12 times per month (2.3±2.3). All said that they occasionally used both drugs in combination and admitted having driven at least once under the influence of each one separately. However, only three admitted to having driven under the combined influence of both. Subjects were paid NLG 650 (ca. US$ 335) upon completion of the study.

2.2 Legality and Ethics

The study's protocol was reviewed and approved in sequence by the District Attorney and the standing Medical Ethics Committee of Maastricht University. Subjects were treated according to the international convention governing drug studies with human volunteers; i.e., the Declaration of Helsinki ( 1964), and its subsequent amendments.

2.3 Design, Doses and Administration

The study followed a balanced, 6.way (2x3 factorial) observer. and subject.blind, placebo controlled, cross.over design. Treatment orders were randomly assigned from those residing in three, 6x6, Williams Squares.

Subjects began treatments by drinking alcohol or alcohol placebo. They continued by smoking marijuana placebo or marijuana delivering THC in doses of 100 or 200 ug/kg. All six combinations of alcohol and THC were consumed by all subjects on separate occasions.

These treatments are respectively designated as shown in Table 2.1.

Alcohol dosing was designed to achieve a peak BAC of 0.06.0.07 g/dl before smoking and 0.04.0.05 g/dl during the driving tests. To achieve this, subjects ate two sandwiches while drinking the initial dose; i.e., 0.6 g/kg of "pure" (99.8%) ethanol mixed with orange juice to a volume of 300 ml and flavored with Grand Cornier essence for masking purposes. This was accomplished within 30 min. Subjects' BACs were monitored at 10 min. intervals for 30.60 min. after the cessation of drinking using a Lion S.D4 Breath Alcohol Analyzer. Those failing to achieve the expected peak BAC were given a booster dose of 0.05.0.2 g/kg in the same proportion to the mixer, whereas the others were given the mixer alone. A second booster dose was given midway through the driving tests in almost all cases for sustaining the desired BAC. Flavored orange juice was given at the same times and in approximately the same volumes in the placebo alcohol conditions. Smoking followed the cessation of the first alcohol dose by 60 min. and continued for the following 10 min. The cigarettes were prepared beforehand for each individual from stock provided by the US National Institute on Drug Abuse. Originally, placebo cigarettes (i.e., containing marijuana leaf from which THC had been removed by ethanol extraction) and those containing the drug were all 85 mm in length and 25 mm in circumference, weighing about 800 ma. Cigarettes with THC concentrations of 2.2% and 3.95% were respectively used for providing 100 and 200 1lg/kg doses. These were cut to provide lengths appropriate for the subjects' weights. Placebo cigarettes were similarly shortened. All were humidified over a saturated sodium chloride solution at 20°C for 24 be before consumption. Subjects smoked them as completely as possible through a plastic holder in their customary fashion.

2.4 Procedures

Two subjects were tested per day. They were transported by an investigator to and from the sites for treatment and testing. After arriving at 19:00 h, subjects yielded breath and urine samples for confirming their compliance with prohibitions against prior use of alcohol and drugs (below). Drinking, followed by smoking, proceeded until 20:40 hr. Thereupon the pair of subjects was taken to the origin of the driving tests. There were two tests (below) that were performed twice per session, all during darkness and in actual traffic. Each member of the pair of subjects began performing a different test. One departed from the origin 2 min. before and the other 2 min. after 21:00 hr. Both proceeded with their initial test assignments while driving over the same 40 km (24.8 mi) highway segment until arriving at the highway exit that marked the terminal, where the first subject awaited the second. When that occurred, their test assignments reversed. The first subject resumed driving at 21:30 h, crossed and re.entered the highway traveling in the opposite direction than before while performing the opposite test. The second did the same after a 4 min. delay. Both paused for 15 min. after returning to the origin. Their BACs were monitored and a booster alcohol dose was given, if needed. Beginning around 22:15 h, the subjects drove through another circuit while repeating the same series of tests as before. Subjects rated their levels of intoxication at the beginning and. end of the entire sequence, and the quality of their performance, at the end of each driving segment. An instructor accompanying the subjects similarly rated their performance at these times. The same subjects always undertook the two driving tests in the same order.

Successive test sessions were ordinarily scheduled for particular subjects at weekly intervals. They were forbidden to smoke marijuana or hashish outside of the study, or to take any other illicit drug, from 7 days before their first session until the conclusion of the last. They were told that the detection of any drug in urine samples provided at the beginning of a session would cause their immediate dismissal. They were similarly forbidden to drink alcohol for 24 be before sessions. They were instructed to retire for and arise from sleeping at normal times and to avoid strenuous physical activities over the day before sessions. Consumption of beverages containing caffeine and of solid food was prohibited for 2 be before session. And, smoking; nicotine containing cigarettes was prohibited during the sessions. Finally, the subjects were prohibited from participating in another biomedical investigation, particularly if it involved drug taking; and, they were required to report the use of any systemic medication taken for personal reasons.

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Part 3 of 4

2.5 Driving Tests and Rating Scales

General Driving Procedures. Subjects operated a specially instrumented Volvo 240 GL station wagon accompanied by a licensed driving instructor having access to redundant vehicular controls from his position in the front passenger's seat. They were told that the instructor's primary role was to ensure test safety but that they would be held responsible under Dutch Law for any collisions that might occur. They were further told that they were legally responsible not to undertake a test and to stop any in progress, if in doubt concerning their ability to drive safely. They were advised that the instructor could require them to bring the vehicle to a halt on the road shoulder if in his opinion their performance was becoming unsafe. No penalties or censure were attached to the subject's decision to avoid or stop driving. Subjects were trained to perform both driving tests in one complete rehearsal of all procedures they would later encounter after drug treatments. Their performance during training rides was evaluated by the instructor and judged normal in every case. Data recorded during treatment sessions were edited and reduced by assistants using interactive computer programs. These personnel were not informed of the nature of the treatments given on particular occasions.

Road Tracking. The standardized version of this test (OHanlon, 1984) involves driving over a 100 km (62 mi) highway circuit with a short interruption for reversing the direction of travel at the mid.point. It normally lasts about one hour. The test was modified for the purposes of the present study. It was given in two, 25 min. parts, separated by an interval of 45 min. Otherwise the test followed the standard model. It was given over that segment of the 4.lane, divided, highway (A2) running north/south between the cities of Maastricht and St. Joost. The subject entered the highway and accelerated to achieve a speed of 100 km/in (62 mph) in the right (slower) traffic lane. His/her instructions were to maintain that speed and a steady lateral position between the delineated lane boundaries over the entire segment. The subject was only allowed to deviate from these instructions for overtaking a slower vehicle traveling in the same traffic lane. Upon arrival at the end of the segment, the subject drove off the highway at the designated exit, parked the vehicle and awaited the next scheduled activity. The vehicle contained systems for recording speed and lateral position. The former originated from an electromagnetic sensor and pulse generator attached to the drive wheels. Pulses were converted to an analog voltage, proportional to speed between 0 and 120 km/in. An electro.optical lateral position sensor was rack.mounted in protective housing over the left rear corner of the vehicle. Its lens acquired an image of road surface directly behind the vehicle, in effect focusing a 3.m band running at right angles to the direction of travel on a linear array of 256 capacitor.coupled photodiodes. Luminance from the left lane.line was normally the greatest falling upon the array, thus causing the most rapid discharge of one particular diode.capacitor at any given moment. The position of that diode was determined relative to a calibrated null.point by rapid (>100 Hz) electronic scanning. The difference was used to generate an analog voltage proportional to the distance the vehicle was to the right or left of lane.center. Full scale values were obtained when the vehicle was ±1.5 m from lane.center. Taking into account both the asymmetric location of the sensor and the vehicle's width (1.6 m), maximum readings occurred when it was 0.38 m onto the road shoulder or into the adjacent traffic lane. Analog signals were digitized by Burr.Brown data acquisition cards using Labtech Notebook Software at a rate of 4 Hz.

These data were time coded and filed on hard.disk in an on.board computer. Files were copied on diskette and edited off.line to remove parts recorded during passing maneuvers and when the lateral position signal was absent or distorted by noise. Data were preliminarily analyzed by 5.km segments to yield mean and standard deviation values of both parameters; and also, intervals when the vehicle was traveling outside of the assigned traffic lane. The values finally recorded as tests scores over each segment were the following: mean speed and lateral position (MSP, MLP); the square roots of pooled speed and lateral position variances as estimates of each parameter's standard deviation (SDSP, SDLP); and, the relative time out of lane (TOL). SDLP was the primary dependent variable. Normative data provided by more than 600 healthy volunteers (<65 yr) who undertook the standard test under placebo control conditions in the Institute's previous studies have shown that SDLP follows a log.normal distribution with mu±sigma of approximately 3.06±0.19 log base e cm units. Thus, about 99% of the distribution fell between 12.5 and 35.5 cm; i.e., the normal limits. SDLP scores from repeated measurements on the same subjects under control conditions are very reliable; i.e., test.retest coefficients of correlation have typically varied between 0.7 and 0.9 for groups of 16.24 individuals (O'Hanlon et al, 1986, 1995). Moreover, SDLP was sensitive to the effects of all sedating drugs so far studied; e.g., to alcohol in blood concentrations as low as 0.035 g/dl (Vuurman et al, 1996) and to THC in doses at or above 100 1lg/kg (Robbe & O'Hanlon, 1993).

Car Following. The second test was developed in a pilot study (Brookhuis et al, 1987) and has been repeatedly applied in progressively improving versions (Robbe & O'Hanlon, 1993; Ramaekers et al, 1994; Vuurman et al, 1996). It measures the driver's ability to perceive changes in an immediately preceding vehicle's speed and react in a manner maintaining a constant headway. The car following test was likewise given in two parts lasting about 25 min. apiece and separated by an interval of 45 min. The test involves two vehicles traveling in tandem, the first operated by an investigator and the second by the subject. They entered the highway and accelerated together to achieve a constant speed of 100 km/in (62 mph). The subject was instructed to drive 50 m (164 ft) behind the preceding vehicle. The instructor defined this distance for the subject at the beginning of each part of the test but did not correct his/her headway again before coming to the end. Once the subject's headway had stabilized, the investigator operating the preceding vehicle executed a series of speed changes interpolated between 0.5.5.0 min. of constant driving. Speed changes in alternating directions were controlled by computer manipulation of a conventional cruise control system. In each case the vehicle accelerated or decelerated in a constant manner causing its speed to rise or fall by 15 km/in (9.3 mph) over 33 sec. The subject's task was to follow this change as quickly and as accurately as possible by controlling his/her own vehicle's speed using the accelerator pedal.

Depending on the traffic, the number of maneuvers during each repetition of the test varied between 4 and 10 in each direction (i.e., acceleration and deceleration), except in one case where only one maneuver was undertaken. The preceding vehicle's speed was continuously measured and transmitted via telemetry to a receiver on the following vehicle. A discrete signal to indicate the beginning of each maneuver was inserted in that FM transmission. The transmission was demodulated and digitally encoded at 4 Hz in a computer file along with the parallel record of the following vehicle's speed. Headway was also measured directly by a ranging laser system (Sick DSE 2000) from a transmitter/receiver located in the center of the following vehicle's grill. This system emitted 4.5 ms pulses at 40 Hz and registered their reflections from a solid 1.5 x 1.0 m screen mounted vertically on the back of the preceding vehicle. The phase angle between each emission and reflection was measured and converted to an analog voltage, proportional to distance. This signal was sampled, digitized and stored in the same manner as the others. Distance recordings were occasionally incomplete owing to misalignment of the vehicles. Missing data were estimated based on the observed differential vehicle speeds. Data were analyzed off.line using an interactive computer program. The program identified the initiation of each maneuver and when the preceding vehicle's speed began and ended its linear change. Usually the following vehicle's speed rose or fell in parallel after the subject recognized the change. In these cases, the program automatically recorded the driver's reaction time (RT) when the speed of his/her vehicle had changed by 2 km/in in the correct direction. Sometimes however, the driver equivocated at the beginning of the maneuver causing the vehicle's speed to vary erratically. In these cases, the analyst extrapolated backward in time from where the subject's speed was clearly in linear ascent or decent to 2 km/in higher or lower than the last reversal. The difference in time between that point and the beginning of the preceding vehicles speed change was taken as the subject's RT. The distance separating the vehicles at the moment of the preceding vehicle's speed began to change was measured as the initial headway. Parameters measured during maneuvers were mean, standard deviation, minimum and maximum headway (H sub M HSD, H min H max RT and HSD are the most important variables, the former measuring the latency of the driver's initial response and the latter, the precision of headway maintenance after that response. (H sub M is normally measured for possible statistical adjustment of HSD should (H sub M vary between conditions in a given experiment. H max and H min were measured for the first time in this study for exploratory purposes.

Subjective Rating Scales. Subjects rated the degree to which they felt "under the influence" of the given drug or combination on a continuous 100.mm scale. It ranged from "not at all" to "the most I have ever experienced." This rating was made just before driving and immediately after its final conclusion. They also rated the quality of their immediately preceding driving performance after each test segment on a scale from 1 to 10 in 0.25 unit steps. Subjects were told to rate their performance in the manner traditionally used in the Dutch school system. The score of 10 means "perfect." Scores of 6 and above are "passing" to "excellent." And, scores below 6 are "failing" to progressively diminishing degrees. The accompanying driving instructor independently rated the subjects' performance at the same times on the same scale and in the same manner.

2.6 Statistical Analysis

SDLP, the primary dependent variable, was analyzed in sequential steps. The first was a check for Period and 1sr.Order Carryover effects using the SAS (6.09) General Linear Model (GLM) procedure with Type I sums of squares. Then Multivariate Analysis of Variance (MANOVA) was applied according to the SPSS/PC+ procedure for repeated measures to test the main effects of Alcohol, THC, Repetitions and their interactions. Data entering this analysis were weighted by orthogonal polynomial coefficients so that the linear and quadratic components of the THC effect over doses, and the same in interaction with alcohol, could be separately tested. Finally, univariate (i.e., ANOVA) mean.pair contrasts between values recorded in the double.placebo condition and every other one were made using pooled error.variance and the Sequential Bonferroni pa adjustment for multiple comparisons (Overall & Rhoades, 1985). MLP, MSP and SDSP were analyzed the same way. TOL was analyzed the same way after log base 10 transformation to adjust for the expected skew in the raw data. All maneuvers performed by subjects in both repetitions of the Car Following Test were analyzed to yield average values of RT, (H sub M and H sub SD by subject and condition. Average values of H max were calculated for acceleration maneuvers, and of H min for deceleration maneuvers. The same analyses were planned for these parameters but circumstances forced their analysis by other means (below). Subjective parameters were analyzed by MANOVA for the effects of Alcohol, THC, Repetitions and their interactions. The between.groups factor of Rater was added to assess differences between subjects' and instructors' ratings of driving quality.

RESULTS
3.1 Adjustments for Missing Data

Table 3.1 lists the tests which failed to yield useful data by condition, repetition within condition and subject. Data were missing for four (1.85%) planned repetitions of the Road Tracking Test and 25 (11.6%) of the Car Following Test. One of the former could not be undertaken by a subject whose BAC exceeded the acceptable limit at the time. The other failures to record data in the Road Tracking Test were attributable to operator error. These missing data were replaced in the planned analyses by scores recorded during the other repetition of the test on the same nights.

Table 3.1 Road Tracking and Car Following tests which failed to yield useful data by condition, repetition within condition and subject (for definition of treatment conditions, see Table 2.1).

Nearly all failures to record data in the Car Following Test were caused by the subjects' unwillingness or inability to consistently maintain a following distance within the range of the sensor/transmitter system. Though partial data were obtained in some of these cases, they were thought to be too meager for deriving reliable parameter averages. The same procedure for replacing data could not be followed since seven subjects did not provide any useful data in either test repetition after particular treatments. It was not possible to apply the planned analyses to car following data. Instead, these data were combined across both test repetitions within each condition to yield average parameter values. They were analyzed in repeated.measures, 2.tailed, l.tests for making separate comparisons between double placebo and every drug condition. The Sequential Bonferroni procedure was again used for adjusting the p-sub alpha criteria for multiple comparisons within each set of five.

3.2 Blood Alcohol Concentration (BAC) / THC (ug/kg)

Subjects' BACs generally peaked in the range of 0.04.0.09 g/dl (mean +/- SD 0.067 +/- 0.015 g/dl) within 1 hour after drinking. Those whose peak BACs were below 0.06 g/dl were given the first booster dose just before marijuana smoking began at 20:30 hr. Their BACs were again measured before beginning the driving tests at 21:00 hr. Descriptive statistics are given in Table 3.2 for that and subsequent measurements which concluded after driving at 23:15 hr.

Table 3.2 Mean+ SD, and range, for BACs ( g/dl) measured in each THC condition at fixed times before, during and alter driving tests. Repetitions of the 25.mint driving tests started at 21:00, 21:30, 22:15, and 22:30 hours.

Subjects began driving in every THC condition with mean BACs close to the legal limit of 0.05 g/dl. Mean BAC declined to about 0.035 g/dl over the course of the next hour. The second booster doses were then administered to subjects with BACs below 0.05 g/dl, which arrested but generally did not reverse the decline. They achieved a mean BAC of about 0.04 g/dl 30 min. Iater and finished driving with about 0.035 g/dl. Thus, most of the subjects performed the tests while their BACs fluctuated around 0.04 g/dl in a generally declining trend from about 0.05 to 0.035 g/dl.

3.3 Road Tracking

Standard deviation of lateral position (SDLP) was the study's primary outcome variable. Preliminary analyses of the SDLP data using the SAS General Linear Models Procedure revealed no significant Period or Carryover effects (F5,75=1.58 & 1.09, respectively), thereby allowing the main analysis to proceed as planned. The results are summarized in Table 3.3. Mean+SE SDLP values recorded in both repetitions within every treatment condition are shown in Figure 3.1.

Subjects drove with the lowest mean SDLP after double placebo (i.e., in 00). The overall effects of both Alcohol and THC were highly significant as was the linear increase in SDLP over THC doses. Every drug treatment significantly elevated SDLP, relative to double placebo, in separate mean.pair comparisons. The main effect of Repetitions was also significant: subjects generally drove with higher SDLPs in the second test repetition than the first. However, the interactive effects of Alcohol, THC and Repetition were uniformly not significant.

Subjects generally drove with SDLPs below the established normal upper limit of 35.5 cm. Two subjects (#11 and #17) exceeded the limit in both AT' and AT2. The first subject drove with SDLPs of 40.3 and 40.4 cm and the second subject with 37.3 and 43.3 cm in these conditions; i.e., in all cases well beyond the 99th percentile driver in the normal population.

Figure 3.1 Mean (±SE) Standard Deviation of Lateral Position (SDLP, cm) in first and second repetitions of the Road Tracking Test in every condition (for definition of treatment conditions, see Table 2.1).

Time out of lane (TOL) was a secondary but important measure of the subjects' road tracking performance. TOL is a natural nonlinear correlate of SDLP. The former is known to be less sensitive to small drug effects but its rise with large ones has more obvious practical implications. Results of the analysis of log base 10 TOL are summarized in Table 3.4.

Table 3 4 Summary MANOVA/ANOVA and Mean.Pair Comparisons for log base 10 TOL (for definition of treatment conditions, see Table 2.1).

Geometric mean +/- SE TOL is shown in Figure 3.2 for every condition and repetition within conditions. Geometric values, or the antilogs of mean±SE log base 10 TOL, are shown in the figure since these better represent central tendencies when the raw data are positively skewed; and, they reflect the mean differences that were actually tested for significance. Most subjects (i.e., 11) occasionally allowed the vehicle's lateral motion to exceed lane boundaries while driving after placebo but the mean percentage of data recorded during these excursions was very low (i.e., 0.26%). The overall effects of Alcohol and THC, though not Repetitions, were significant. Mean.pair comparisons showed that the significant main effects were mainly attributable to the drug combinations. Whereas neither alcohol alone nor THC 100 ~g/kg alone had appreciable effects, and THC 200 1lg/kg alone had an effect that only approached significance (P=0.077), the two combinations very significantly elevated TOL. Alcohol plus THC 100 ug/kg. caused mean TOL to rise above 0.6%, and alcohol plus THC 200 1lg/kg, to about 1.1%. Nevertheless, the interactive effect of alcohol and THC was not significant.

Figure 3.2 Geometric mean (±SE) of Time out of Lane (TOL, %) in first and second repetitions of the Road Tracking Test in every condition (for definition of treatment conditions, see Table 2.1).

Standard deviation of speed (SDSP) was not significantly affected by any factor. Mean lateral position (MLP) and mean speed (MSP), control variables for ascertaining whether subjects followed their instructions, were similarly unaffected by treatments. Both showed slight effects of Repetitions: on the average, MLP shifted from 8.23 to 10.03 cm to the right of mid.line (F sub 1,17 = 5.25; P=0.035) and MSP rose from 97.4 to 97.7 krn/h (F sub 1,17 = 3.84; P=0.067). More importantly for control purposes, all individuals drove with MLPs between 30 cm left, and 41 cm right of mid.line and with MSPs between 91.2 and 101.8 km/hr. It seems they generally attempted to drive with a steady lateral position near the center of the traffic lane and a constant speed of 100 km/in (62 mph).

3.4 Car Following

Table 3.5 summarizes the results of l.tests comparing the effects of placebo and every drug treatment on car following parameters averaged separately over acceleration and deceleration maneuvers. No treatment significantly affected any parameter during acceleration maneuvers. Likewise, treatments failed to affect mean headway (H sub M during deceleration maneuvers. Mean values of (H sub M varied irregularly between conditions from 41.9 m in OT' to 44.2 m in OO. The lack of any treatment effect on this parameter is important. If drugs caused the subjects to maintain longer average headways, it would be difficult to interpret differences between their effects and placebo's on the other parameters measured in the test. And as indicated in the table, those differences were abundant during deceleration maneuvers.

Table 3.5 Summary of t.test comparisons of drug and placebo effects on average RT, HSD and H max / H min during acceleration and deceleration maneuvers, separately. Values in the table are mean differences, t and p. Asterisks indicate significant p.values according to adjusted p,,` criteria (for definition of treatment conditions see Table 2.1)

Figure 3.3 shows mean+SE for RT, HSD and HMN in every condition. Mean RT rose from the placebo level after every drug treatment. The mean difference for AT2 was significant, and for OT', OT2 and AT~, nearly so (P<0. 10). All of the mean differences in HSD were significant after Pa adjustment for multiple comparisons. So were differences in HMIN, except that between OO and OT2. Thus, in one, two or all three respects, every drug treatment impaired car following performance relative to placebo.

Figure 3.3 Mean (+ SE) Reaction Time (RT, see), Standard Deviation of Headway (~D, m), and Minimum Headway (HMIN, m) during deceleration maneuvers in the Car Following Test in every condition (for definition of treatment conditions, see Table 2.1).

3.5 Intoxication Ratings

Subjects began the various conditions making estimates of their degrees of intoxication that were reasonable considering the treatments given beforehand. Some at the beginning of the double placebo condition thought they felt something and reported low levels of intoxication (mean+SD, 7.67i2.37%). However, nearly all reported stronger feelings after alcohol and both THC doses, given alone; i.e., 44.56i5.48, 29.11 i4.20 and 36.50i7.10%, respectively. Combinations of alcohol with low and high THC doses produced still higher intoxication ratings; i.e., 52.89i4.66 and 62.87i4.90%, respectively. No subject felt normal after alcohol alone or either combination of Alcohol and THC. Both of the latter produced extreme ratings above 90%. In all conditions, mean intoxication ratings at the conclusion of testing were at levels about half of where they began. As might be expected, both the overall effects of alcohol and THC were highly significant (F,,,7.~ 2,~6=76.04, 19.76; P<0.001). The linear THC dose effect was also significant (F,,,7=41,97; P<0.001). There was a significant Alcohol x THC interaction (F2,6=10.07; P~0.001). However, this finding is mitigated by the failure to find a significant linear component of that interaction (F. ,7=0.78). Instead, the quadratic component was significant (F,,,7=11.52; P<0.003). One can not interpret this interaction as showing that the rise in feelings of intoxication was steeper over THC doses given with alcohol. Rather it reflected the fact that the rise was precipitous from OO to OT' and OT2 but more gradual from AO to ATE and AT2.

3.6 Driving Quality Ratings

Subjects rated their driving quality at the end of every test repetition. Instructors rated the subjects' performance in parallel. A summary of results from analyses comparing the subjects' and instructors' ratings over treatment conditions and repetitions of tests within conditions is given in Table 3.6. MeaniSE ratings by both are shown in Figure 3 '1. Also given are the frequencies of scores indicating that the subjects' driving quality was unacceptable; i.e., that in their own or the instructor's opinion, they had "failed" the test.

Instructors rated the subjects' performance as significantly worse than the subjects did themselves. The mean differences were not large in magnitude and otherwise the two sets of ratings were quite similar. Ratings of the subjects' performance in the Road Tracking Test clearly reflected the separate effects of alcohol and THC; and, THC's linear dose effect. Somewhat surprisingly, those ratings did not show a separate significant effect of alcohol in the Car Following Test. THC's effects and its increase with dosage were, however, also apparent in this context. Both subjects and instructors rated the subjects' performance as generally worse in the first repetitions of both tests. The ratings showed a significant Alcohol by THC interaction. But as with the interaction involving the intoxication ratings (above), the quadratic but not the linear component was significant. Thus the interaction for driving quality ratings must be interpreted in the same manner.

Table 3 . Summary of MANOVA/ANOVA for Subjects and Instructor Ratings of Driving Ouality in Road Tracking and Car Following Tests.

"Failures" occurred more frequently in the instructors' ratings than the subjects'; and, with about equal frequencies in both driving tests. According to the instructors, about half of the subjects' driving was so poor that they failed in AT2 and about one.third in ATE. The failure rate was lower and about the same in OT2 and OT'.

Figure 8 . Mean (+SE) ratings of driving performance in the Road Tracking (top) and Car Following Test (bottom), in each treatment condition, as scored by the driving instnactors (I) and the subjects If). Also shown are the frequencies of subjects failing the particular test.repetition according to themselves (S) and the instructors (I) (for definition of treatment conditions, see Table 2.1).

3.7 Instructor's Comments

Table 3.7 summarizes the instructor's written comments concerning particularly unusual or extreme aspects of the subjects' driving behavior in the various treatment conditions. Only one (#11) drew any comment in the double placebo condition. At the other extreme, 10 subjects drew comments after being treated with the combination of alcohol and THC 200 ~g/kg. Only three, all females (#10, #13, #16), failed to elicit any comment.

Table 3.7 Summary of Instructor's Conunents concerning unusually deviant driving behavior by subjects within conditions. Males identified as subjects 01.09, finales, as 10.18 (for definition of treatment conditions, see Table 2.1).

The data presented in the table are self explanatory, except those in the last three categories. One subject's (#17) behavior was described as reckless in AT2 because she drove at speeds well above 100 km/in (62 mph) while weaving, swerving and abruptly overcorrecting. When repeatedly cautioned by the instructor to maintain the designated speed, she immediately did so but shortly resumed driving as before. Four subjects apparently experienced bizarre memory disturbances, one (#11) in AT, and the others (#04, #06, #14) in AT2. Subject #1 l's disturbance occurred in the Road Tracking Test. She began driving after the departure of the other subject who was performing the Car Following Test. She eventually overtook and proceeded to pass that subject's vehicle on the left, but failed to recognize it or the leading vehicle under an investigator's control. She completed the maneuver by returning to the right traffic lane between the vehicles engaged in this test. She was visibly surprised when the instructor drew the error to her attention. Subjects' #04 and #06 memory lapses were similar: midway through the Car Following Test, both forgot the procedure and attempted to pass the leading vehicle. Another (#14) correctly followed the procedure during the same test but upon its conclusion, failed to leave the highway with the leading vehicle at the designated exit. She apparently would have continued to drive on the highway beyond that point if not corrected by the instructor. Finally, subject #11 in OTT, AT' and AT2, and subject #14 in OT2, had to be prevented from beginning a passing maneuver while their vehicle was in the process of being overtaken by other traffic. It seemed to the instructor that the subjects failed to inspect the rear.view mirrors before initiating these maneuvers.

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