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Marijuana And Actual Driving Performance

U.S. Department of Transportation,
National Highway Traffic Safety Administration
(DOT HS 808 078), Final Report, November 1993



The program proceeded into 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 g/kg. It was given to a new group of 16 regular marijuana (or hashish) users, along with a placebo. For comparative purposes, another group of 16 regular users of alcohol, but not marijuana, were treated with a modest dose of their preferred recreational drug, ethanol, and again placebo, before undertaking the same city driving test. Both groups were equally comprised of men and women.

Marijuana was administered to deliver 100 g/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. Blood samples were taken immediately prior to and following all placebo and drug driving tests (i.e. 20 and 80 minutes after initiation of smoking, or 35 and 95 minutes after initiation of drinking).

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 'molecular' approach adopted from Jones (1978), 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. The second, a 'molar' approach, required the driving instructor acting as the safety controller during the tests to retrospectively rate the driver's performance using a shortened version of the Royal Dutch Tourist Association's Driving Proficiency Test. In total, 108 items were dichotomously scored, as either pass or fail. Total test performance was measured by the percentage items scored as 'pass'. Subscores were calculated for vehicle checks, vehicle handling, traffic maneuvers, observation and understanding of traffic, and turning'. This method has been applied previously to show the impairing effects of alcohol and diazepam (De Gier, 1979; De Gier et al., 1981).


Data from two male subjects in the marijuana group were excluded from the results because neither THC nor THC-COOH was found in their plasma after smoking.

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. The molar approach was more sensitive. Table 1 shows that a modest dose of alcohol (bac=0.034 g%) produced a significant impairment in city driving, relative to placebo. More specifically, alcohol impaired both vehicle handling and traffic maneuvers. Marijuana, administered in a dose of 100 g/kg THC, on the other hand, did not significantly change mean driving performance as measured by this approach.

Subjects' ratings of driving quality and effort to accomplish the task were strikingly different from the driving instructor's ratings. Both groups rated their driving performance following placebo as somewhat better than 'normal'. Following the active drug, ratings were significantly lower (35%, p<.009) in the marijuana, but not (5%, ns) in the alcohol group. Perceived effort to accomplish the driving test was about the same in both groups following placebo. Following the active drug, a significant (p<.033) increase in perceived effort was reported by the marijuana, but not the alcohol group.

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.

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.

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