Cannabis and traffic collision risk: Findings from a case-crossover study of injured drivers presenting to emergency departments

Objectives: This study examined whether acute cannabis use leads to an increased collision risk. Methods: Participants were 860 drivers presenting to emergency departments in Toronto and Halifax, Canada, with an injury from a traffic collision, between April 2009 and July 2011. Cannabis and other drug use were identified either through blood sample or self-report. A case-crossover design was employed with two control conditions: A fixed condition measuring substance use during last time driving, and whether the driver typically uses cannabis prior to driving. Collision risk was assessed through conditional fixed-effects logistic regression models. Results: Results revealed that 98 (11 %; 95 % CI: 9.0-13.1) drivers reported using cannabis prior to the collision. Regression results measuring exposure with blood and self-report data indicated that cannabis use alone was associated with a fourfold increased (OR 4.11; 95 % CI: 1.98-8.52) odds of a collision; a regression relying on self-report measures only found no significant association. Conclusions: Main findings confirmed that cannabis use increases collision risk and reinforces existing policy and educational efforts, in many high-income countries, aimed at reducing driving under the influence of cannabis.