ABSTRACT: THE IMPACT OF CANNABIS ON DRIVING

Background: Cannabis is known to have detrimental effects on human
performance and may also affect driving adversely. However, studies
designed to examine this issue have provided equivocal findings. We set up
this study to further determine the effect of cannabis on driving.

Methods: We used a cross-sectional, case-control design with drivers aged
20-49 who were involved in a fatal crash in the United States from 1993 to
2003; drivers were included if they had been tested for the presence of
cannabis and had a confirmed blood alcohol concentration of zero. Cases
were drivers who had at least one potentially unsafe driving action
recorded in relation to the crash (e.g., speeding); controls were drivers
who had no such driving action recorded. We calculated the crude and
adjusted odds ratios (ORs) of any potentially unsafe driving action in
drivers who tested positive for cannabis but negative for alcohol
consumption. In computing for the adjusted OR, we controlled for age, sex,
and prior driving record.

Results: Five percent of drivers tested positive for cannabis. The crude OR
of a potentially unsafe action was 1.39 (99% CI = 1.21-1.59) for drivers
who tested positive for cannabis. Even after controlling for age, sex, and
prior driving record, the presence of cannabis remained associated with a
higher risk of a potentially unsafe driving action (1.29, 99% CI = 1.11-1.50).

Conclusion: Cannabis had a negative effect on driving, as would be
predicted from human performance studies. This finding supports the need
for interventions to decrease the prevalence of driving under the influence
of cannabis, and indicates that further studies should be conducted to
investigate the dose-response relationship between cannabis and safe driving.