Dr G.B. Chesher
Department of Pharmacology University of Sydney and National Drug and
Alcohol Research Centre University of New South Wales.
8. SUMMARY (OF THE EVIDENCE PRESENTED ABOVE)
|The most recent of the reports of studies of the effects of cannabis on
actual driving performance included a summary of the published literature on marijuana and
driving. They concluded this review with the following paragraph:
|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. To a large extent, the results from driving simulator
and closed-course tests corroborate the epidemiological findings by indicating that THC in
single inhaled doses up to 250 µg/kg has relatively minor effects on driving performance,
certainly less than BACs in the range of 0.08Ę-Ę0.10g%.
|Apart from the above, a very important finding in the reviewed studies is
the difference in the drug users' awareness of the effect of the drugs alcohol and
cannabis. Alcohol use is accompanied by increased confidence, an impairment of judgement
to the extent that driving behaviour becomes more risky, with faster speeds and a greater
willingness to take risks. Cannabis use on the other hand, is accompanied by compensatory
driving behaviour, including a reduced willingness to take risks and slower driving
speeds. Indeed the compensation was described by Robbe and O'Hanlon in the following
|Very importantly our city driving study showed that
drivers who drank alcohol overestimated their performance quality whereas those who smoked
marijuana underestimated it. Perhaps as a consequence, the former invested no special
effort for accomplishing the task whereas the latter did, and successfully. This evidence
strongly suggests that alcohol encourages risky driving whereas THC encourages greater
caution, at least in experiments.
|The task of driving has been described as a 'self-paced' task. That is,
drivers choose their own levels of task difficulty. There is a difference therefore
between a driver's skills performance, as measured in individual laboratory tasks
and driver behaviour. Driver performance, or skills performance is what a driver can
do. Driver behaviour is what a driver actually does. Driving skills (or
driver skills performance) differ very widely within a community. Some of us may be
extremely cautious and others much less so. The correlation between driver skills and
crash probability is not as great as many may imagine. For example, it is held by many
that superior driver skills lead to reduced crashes and this led to the concept of
'advanced driver training'. Indeed, an editor of a road magazine claimed: 'I have for many
years claimed that the licensed racer is far safer than ordinary chaps, on the grounds of
practised skills, mental ability, cognisance of hazards in driving, keen interest in
driving as well, and so on.'
|In order to examine the possibility that unusually skilled drivers really
did have different on-the-road driving records from the average driver, a comparison was
made of the on-the-road driving records of a group of licensed racing drivers with those
of other drivers matched for such characteristics as sex and age, etc. What they found was
that in all measures of traffic violations including crashes, speeding violations, other
moving violations as well as non-moving violations, the rates for the racing drivers
exceed those of the comparison drivers, in most cases by a considerable margin.
|In the light of the above, Terhune et al. asked the following
|A nagging question which qualifies conclusions from
epidemiological studies of drugs in crashes is: If certain drugs are linked to elevated
crash risks, how much of the elevation is due to characteristics of the people who use
|For example, Terhune in a literature review remarked that research
revealed a striking similarity between the personal correlates of marijuana use and the
correlates of crash involvement. Rebellious, deviant, youthful males were prominent among
marijuana users and among those in crashes. Jessor et al. also addresses these
|A general conclusion made by Robbe and O'Hanlon when discussing the
results of their study and of their review of the literature is worth citing here as a
general conclusion to this review:
|In summary, this program of research has shown that
marijuana, when taken alone, produces a moderate degree of driving impairment which is
related to the consumed THC dose.
|The impairment manifests itself mainly in the ability to
maintain a steady lateral position on the road, but its magnitude is not exceptional in
comparison with changes produced by many medicinal drugs and alcohol.
|Drivers under the influence of marijuana retain insight
in their performance and will compensate where they can, for example, by slowing down or
increasing effort. As a consequence THC's adverse effects on driving performance appear
relatively small. Still we can easily imagine situations where the influence of marijuana
smoking might have an exceedingly dangerous effect ie, emergency situations which put high
demands on the driverŐs information processing capacity, prolonged monotonous driving,
and after THC has been taken with other drugs especially alcohol.
|We therefore agree with Moskowitz's conclusion that 'any
situation in which safety both for self and others depends on alertness and capability of
control of man-machine interaction precludes the use of marijuana'.
|However, the magnitude of marijuana's relative to many
other drugs' effects also justify Geringer's (1988) conclusion that 'marijuana impairment
presents a real, but secondary, safety risk; and that alcohol is the leading drug-related
risk factor'. 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.