Is driving under the influence of cannabis becoming a greater risk to driver safety than drink driving? Findings from a longitudinal study.

The present study examined the associations driving under the influence of (a) cannabis and (b) alcohol, and motor vehicle collisions during, in a longitudinal study of a New Zealand birth cohort (n=936). Participants reported significantly (p<.0001) greater rates of driving under the influence of cannabis than driving under the influence of alcohol during ages 21-25. Also, there were statistically significant bivariate associations between increasing levels of both: (a) driving under the influence of cannabis and (b) self-reported driving under the influence of alcohol, and increased risks of active motor vehicle collisions (p<.0001). These associations were adjusted for potentially confounding factors including average distance driven and self-reported risky driving behaviours. After adjustment, the associations between driving under the influence of cannabis and motor vehicle collisions remained marginally significant (p=.064), whereas adjustment for confounding factors reduced the association between driving under the influence of alcohol and motor vehicle collisions to statistical non-significance (p>.70). The results of the present study suggest that, for some populations, the risks of driving under the influence of cannabis may now be greater than the risks of driving under the influence of alcohol.