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References on Drugs and Driving

Marijuana And Actual Driving Performance

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



The first driving study was conducted on a highway closed to other traffic. 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 g/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 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 medicinal drugs and alcohol. Other dependent variables were mean speed, and standard deviations of speed and steering wheel angle. Blood samples were taken 10 minutes before the driving tests (i.e. 30 and 90 minutes after initiation of smoking, respectively).


All subjects were willing and able to finish the driving tests without great difficulty. Data from one male subject were excluded from the results because no drug was found in his plasma after smoking.

Figure 1 demonstrates that marijuana impairs driving performance as measured by an increase in lateral position variability: all three THC doses significantly affected sdlp relative to placebo (p<.012, .001 & .001, for the 100, 200 & 300 g/kg conditions, respectively. The Dose by Time effect was not significant indicating that impairment after marijuana was the same in both trials. Marijuana's effects on sdlp were compared to those of alcohol obtained in a very similar study by Louwerens et al. (1987). It appeared that the effects of the various administered THC doses (100-300 g/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. Plasma concentrations of the drug were clearly related to the administered dose and time of blood sampling but unrelated to driving performance impairment.

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