«The results of studies within the last 10 years have failed to present clear evidence for a role of cannabis in road crashes. The role of alcohol in all studies has proved to be dominant.
The evidence indicates that there is a clear difference in the mode of action of cannabis and alcohol, both pharmacological and behavioural and this is presented and the implications described.
The most recent of studies of cannabis and driving (Robbe & O’Hanlon, 1993), which was sponsored by the U.S. National Highway Safety Traffic Administration included a review of the literature. The authors’ comments in summary of their literature review and of their own results include the following:
The foremost impression one gains from reviewing the literature is that no clear relationship has ever been demonstrated between marijuana smoking and either seriously impaired driving performance or the risk of accident involvement. The epidemiological evidence, as limited as it is, shows that the combination of THC and alcohol is over-represented in injured and dead drivers and more so in those who actually caused the accidents to occur. Yet there is little if any evidence to indicate that drivers who have used marijuana alone are any more likely to cause serious accidents than drug free drivers.
Of the many psychotropic drugs, licit and illicit, that are available and used by people who subsequently drive, marijuana may well be among the least harmful.»