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|Major Studies of Drugs and Drug Policy|
|Canadian Senate Special Committee on Illegal Drugs|
|Volume I - General Orientation|
Epidemiological studies indicate a relatively high level of driving under the influence of cannabis, between 5% to 12% of drivers, mostly among young men. At the same time, neither these studies nor the responsibility/risk analyses reach clear conclusions concerning the role of cannabis in dangerous driving. Hence the interest in studies on how cannabis affects driving ability and driving itself. Studies on the psychomotor and cognitive skills needed to drive vehicles have measured factors such as: motor coordination, reaction time, attention, visual attention and deductive reasoning. There are two types of studies on driving: simulated studies and field studies, whether on a track, in the city or on a highway. Most studies focus on single doses for recreational users. They use control group protocols and cross-linked protocols, including placebos and comparisons with alcohol. However, they are limited by the fact that they mainly measure the acute effects of single doses, making it difficult to determine whether more experienced users would react in the same way. The following sections examine both types of study.
In 1985, Moskowitz published a remarkable synthesis of studies on the psychomotor and cognitive effects of cannabis. In this synthesis, he examined motor coordination, reaction time, tracking and sensory functions. The author observed the following:
Ramaekers et al. (2002), reported a meta-analysis on 87 controlled laboratory studies on the psychomotor effects of cannabis conducted by Berghaus et al. (1998). These authors found that the number of psychomotor functions linked to driving (following, reaction time, perception, hand-eye coordination, body balance, signal detection and divided and continuous attention) affected by THC reached a maximum during the first hour after smoking, and one to two hours after oral ingestion. The maximum figures were comparable to those obtained with an alcohol concentration equivalent to > 0.05 g/dl. The number of functions affected reached zero after three to four hours, and only higher doses continued to have an effect. The studies surveyed also showed that THC concentration in the blood is highly correlated to psychomotor effects: a concentration of between 14 ng/ml and 60 ng/ml affected between 70% and 80% of tasks.
The following table summarizes these data:
More recently, after surveying the studies carried out in recent years, the reports prepared by INSERM and the International Scientific Conference on Cannabis reached largely similar conclusions: cannabis affects reaction time where choice is involved, road tracking, shared attention and continuous attention, as well as memory processes, but does not significantly affect simple reaction time or visual or eye-movement functions.
One of the weaknesses of the laboratory studies is the difficulty of relating psychomotor and cognitive tasks directly to driving. Several tests measured in these studies are short and relatively simple and do not necessarily reflect real situations. The advantage of simulated driving studies and field driving studies is that it brings the conditions closer to reality.
Most contemporary studies have similar characteristics: subjects have had a driver’s licence for at least three years. They are often regular cannabis users. The subjects receive either cannabis or a placebo in a double-blind situation that is very strictly timed to control the level of THC transmitted. In some instances, the experimenters also include comparisons with alcohol and an alcohol placebo. However, it is impossible to control how much subjects inhale and actually absorb. The cannabis prepared by the U.S. National Institute of Drug Abuse (NIDA) varies between 1.75% THC for low doses, 2.67% for moderate doses and 3.95% for strong doses. Converted into mg/kg of weight, the doses correspond to 100, 200 and 300 mg/kg, whereas the heavy dose usually preferred by regular users is generally 308 mg/kg. The subjects are familiarized with the equipment used and the tasks to be performed, and are accompanied by instructors on actual driving studies. Measurements include the standard deviation of lateral position in relation to the road, the control over longitudinal position (distance) in relation to the vehicle ahead, decision-making in emergencies, style of driving and risk taking.
The following table, adapted from INSERM data, summarizes a number of the more recent studies.
It is interesting to recall that one of the first driving studies on the road was conducted for the Le Dain Commission. In this study, on a closed track, 16 subjects were each given the 4 following preparations: placebo, marijuana 21 and marijuana 88 μg/kg THC and a dose of alcohol equivalent to BAC 0.07. The tests were conducted immediately after use and three hours later. The subjects were to complete six circuits of the track (1.8 km) with manoeuvres involving slowing down while going forward and backwards, maintaining a trajectory and weaving through cones. The alcohol and heavy dose of marijuana decreased driver performance in tests conducted immediately after use. At the heavy cannabis dose, drivers drove more slowly. On the second test, the differences were less clear.
When the results of this study are compared to those conducted more recently using much more sophisticated methods, it can be seen that the results are remarkably similar. Thus the following was observed:
Lastly, with low doses, subjects had the impression that their driving was not as good as observers felt it was, which was not necessarily the case with higher doses, where the perceptions of both the drivers and the observers agreed.
 Moskowitz, H., (1985) “Marihuana and Driving.” Accid. Anal. Prev., 17 (4), pages 323-345.
 Ibid., page 330.
 Ramaekers J.G. et al. (2002), op. cit., page 77.
 Table adapted from INSERM (2001) op. cit., pages 183-184.
 See Hansteen, R.W, et al. (1976) “Effects of cannabis and alcohol on automobile driving and psychomotor tracking.” Annals of the New York Academy of Science, 282, pages 240-256.
 See notably the survey of studies and the discussion in Smiley, A., (1999) “Marijuana: On-Road and Driving Simulator Studies” in Kalant, H. et al., (ed) The Health Effects of Cannabis. Toronto: Addiction Research Foundation, pp. 173 passim.
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Major Studies of Drug and Drug Policy
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Licit and Illicit Drugs
Short History of the Marijuana Laws
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LSD, Mescaline, and Psychedelics
Drugs and Driving
Children and Drugs
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American Society for Action on Pain
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Medical Marijuana Throughout History
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