Marijuana's impairing effects on driving are moderate when taken alone but severe when combined with alcoholH. Robbe *
Maastricht University, Maastricht, The Netherlands
*Correspondence to H. Robbe, Bakken Research Center, P.O. Box 1220, 6201 MP Maastricht, The Netherlands.
Keywords
marijuana • cannabis • alcohol • driving performance • impairment
Abstract
Previous experimental and epidemiological studies failed to provide unequivocal evidence that marijuana, either alone or in combination with alcohol, impairs a driver's performance to the extent that it will compromise traffic safety.
We investigated the effects of marijuana, alone and in combination with alcohol, on actual driving in four, single-blind, randomized, cross-over studies. In Study 1, 24 subjects performed a road-tracking test on a closed segment of a primary highway after smoking marijuana that contained 0, 100, 200 and 300 g/ kg 9-tetrahydrocannabinol (THC).
In Study 2, 16 new subjects smoked the same THC doses before they performed a road-tracking and a car-following test; however, this time in the presence of other traffic.
In Study 3, two groups of 16 subjects performed a city driving test. One group smoked marijuana delivering 0 and 100 g/ kg THC prior to driving; the other group drunk orange juice mixed with or without a low dose of alcohol.
In Study 4, 18 subjects performed a road-tracking and a car-following test in each of six conditions where they smoked marijuana with 0, 100, or 200 g/ kg THC after they had consumed orange juice with or without alcohol.
In these studies, marijuana alone significantly increased lateral position variability in the road-tracking test and distance variability during deceleration manoeuvres in the car-following test.
Reaction times during car-following were not significantly affected, and a THC dose of 100 g/ kg did not impair city driving performance.
Blood plasma concentrations of THC and THC-COOH were not related to the degree of impairment.
A low dose of alcohol (i.e. blood alcohol concentrations around 0·04%) impaired performance in all driving tests.
Whereas marijuana's effects on driving performance were small (100 g/ kg THC) or moderate (200 and 300 g/ kg) when taken alone, they were severe when combined with a low dose of alcohol.
In conclusion, marijuana alone impairs driving performance, with the degree of impairment increasing from small to moderate as the THC dose increases from 100 to 300 g/ kg.
However, when low to moderate doses of THC (100 and 200 g/ kg) are taken in combination with a low dose of alcohol sufficient for attaining a BAC of about 0·04% actual driving is severely impaired.
http://www3.interscience.wiley.com/journal/4287/abstract?CRETRY=1&SRETRY=0