Cannabis and traffic collision risk: findings from a case-crossover study of injured drivers presenting to emergency departments.
OBJECTIVE
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.
Department of Community Health and Epidemiology, Centre for Clinical Research, Dalhousie University, 4th Floor, 5790 University Avenue, Halifax, NS, B3H 1V7, Canada, mark.asbridge@dal.ca.
Mark
M
Asbridge
Robert
R
Mann
Michael D
MD
Cusimano
Cynthia
C
Trayling
Michael
M
Roerecke
John M
JM
Tallon
Alyce
A
Whipp
Jürgen
J
Rehm
D000063:Accidents, Traffic / Q000517:prevention & control / Q000706:statistics & numerical data*; D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D018592:Cross-Over Studies; D004636:Emergency Service, Hospital; D005260:Female; D006801:Humans; D008297:Male; D008385:Marijuana Smoking / Q000009:adverse effects*; D008875:Middle Aged; D009674:Nova Scotia / Q000453:epidemiology; D009864:Ontario / Q000453:epidemiology; D036301:Qualitative Research; D018570:Risk Assessment; D014947:Wounds and Injuries / Q000453:epidemiology / Q000209:etiology; D055815:Young Adult