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1. EXECUTIVE SUMMARY |
| There is no doubt that cannabis, smoked or taken by mouth produces a
dose-related deficit in tests of performance skills as conducted in a laboratory. |
| Using driving simulators and on-road real vehicles, cannabis has been
shown to affect driving performance. However, the effects are less severe than would be
anticipated from the evidence obtained from the laboratory studies of individual tests of
skills performance. |
| A description is given of epidemiological
studies to determine the role of cannabis in road crashes. The
pharmacological problems associated with these studies are
described. 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. Campaigns to discourage the use of marijuana by drivers are certainly
warranted. But concentrating a campaign on marijuana alone may not be in proportion to the
safety problem it causes. |
