Cannabis and Motor Vehicle Crashes

Cannabis, the second most widely used recreational drug in the world after alcohol, impairs the skills required for safe driving and increases the risk of car crashes. Cannabis legalization in Canada may result in more cannabis related motor vehicle crashes (MVCs). The CIHR funded Cannabis and Motor Vehicle Crashes (CMVC) study uses a culpability analysis approach to investigate the risk of crashing associated with cannabis by studying the association of Δ-9-tetrahydrocannabinol (THC) levels in injured drivers with the likelihood of having caused the crash. Since 2011, this study has collected data on the prevalence of cannabis, alcohol, and other drugs in just over 3000 moderately injured drivers in British Columbia. The results of the culpability analysis on the association between acute cannabis use and risk of crash using data from the first 3000 drivers were recently published.

Led by Dr. Brubacher, the CMVC study is ongoing to evaluate changes in the prevalence of cannabis use in injured drivers following the legalization of cannabis, with the goal of informing public policies targeting cannabis impaired driving. Participants are recruited from four BC trauma centres serving a mixed rural/suburban/urban patient population: Victoria General Hospital (Vancouver Island), Vancouver General Hospital (Lower Mainland), Royal Columbian Hospital (New Westminster, Fraser Valley), and Kelowna General Hospital (BC’s Interior).

Our overarching aim is to provide relevant data to inform policy for cannabis impaired driving in Canada. The following are our study objectives:

1. To study changes in substance use among injured drivers following legalization of recreational cannabis. To do this, we will investigate pre vs post legislation changes in:

  • The prevalence of injured drivers who test positive for cannabis.
  • The mean THC concentration (in THC positive drivers).
  • The prevalence of injured drivers with blood alcohol levels (BAC) > 0, and with BAC > 0.08% (0.08% is the Canadian legal limit for BAC in drivers). Fewer drinking drivers might indicate a substitution effect whereby potential drinking drivers use cannabis instead of alcohol.

2. To refine estimates of the relationship between THC level and the likelihood of being responsible for a crash, with the aim of establishing an evidence-based per se THC level for driving.


Funded By:








Brubacher, J. R., Chan, H., Erdelyi, S., Macdonald, S., Asbridge, M., Mann, R. E., … & Schreiber, W. E. (2019). Cannabis use as a risk factor for causing motor vehicle crashes: a prospective study. Addiction.Download Here 

Brubacher, J. R., Chan, H., Erdelyi, S., Asbridge, M., Mann, R. E., Purssell, R. A., & Solomon, R. (2018). Police documentation of drug use in injured drivers: Implications for monitoring and preventing drug-impaired driving. Accident Analysis & Prevention, 118, 200-206. Download Here

For more information please contact research associate, Herbert Chan at