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, 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 :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 minutes 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/h (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 manoeuvres. Both THC doses alone, and alcohol alone, significantly impaired the subjects' Road and Tracking and Car Following performances. 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. While the effects of THC alone in doses up to 200 :g/kg might be categorised as "moderate", they become "severe" when THC is combined with a moderate dose of alcohol. (Author/publisher)
Abstract