Cannabis and Driving
Reviews of the scientific literature suggest cannabis may impair performance on certain psychomotor tasks. Cannabinoids are found in an increasing percentage of fatal accident victims. Studies of driving simulators, actual driving behaviour, or culpability analysis applied to accidents demonstrate little impairment of performance compared to the effects of alcohol, some studies even suggest improvements in driving behaviour.
IDMU conducts annual surveys via anonymous questionnaires studying drug consumption patterns and other questions relating to psychotropic drug use. Respondents in 1994 and 1998 were asked to state the number of accidents over the previous 5 years, and (1998) the average distance driven per year. Accident rates were compared with estimates of accident frequency from DETR and motor insurance claim statistics. Weighting factors included driving experience, mileage driven, age and sex.
Results: Overall, drug using drivers reported 9% fewer accidents than would be expected from a comparable population group. However, younger drug-using drivers showed increased risks compared to their peers, whereas drivers over 25 reported decreased incidence of accidents. Notable sex differences included drug-using women under 25 or over 40 reporting more accidents than their peers, whereas only males under 20 reported increased risks, with males over 25 reporting markedly lower risk.
Interpretation: Impairment of driving ability from cannabis appears most pronounced among young drivers, suggesting tasks requiring conscious thought or learning may be most affected, whereas "automatic" tasks may be less susceptible to disruption. The overall effect of cannabis on driving suggests a trade-off between mild psychomotor impairment and improvement in driving behaviour. In males, improved behaviour may outweigh any psychomotor impairment, whereas in females there may be less scope for improvement in behaviour, drug-impairment may outweigh marginal behavioural improvements.
Recommendations: Current police and forensic procedures commonly fail to establish intoxication or impairment of driving ability. There is an urgent need for a non-invasive roadside test for presence of active drug, rather than metabolites, and for objective tests of impairment of driving ability.
Atha MJ, Blanchard S & Davis S (2000)
Acknowledgements: The authors wish to acknowledge the help and practical support of the following: Hemp Expo, CLCIA, Green Party Drugs Group, Linda Hendry, Howard Marks, Weed World & Bush Telegraph magazines for survey distribution. We are also grateful to Dr Alison Smiley (University of Toronto), the University of Adelaide Dept of Clinical & Experimental Pharmacology and Ms Gazala Akram for source materials and helpful comments Special thanks are due to the thousands of drug users who completed the questionnaires.
© IDMU publications April 2001
"I was very impressed... it was an excellent report" - Dr R J Tunbridge @ Transport Research Laboratory
"This is the most comprehensive review that I have seen on this vexed topic, and your own IDMU data are of course very valuable also." - Professor Leslie Iverson @ Oxford University - Pharmocology Department
"Research studies such as this are most welcome in helping us better understand and address this problem....I am grateful to you for sending me this paper, which I found most interesting" - Keith Hellawell QPM , UK Anti Drugs Co-ordinator.