Is Cannabis Legalization Associated with an Increase in Cannabis-related Motor Vehicle Crash Fatalities?

Cannabis use is a risk factor for motor vehicle crash (MVC) fatalities, but the degree of a driver’s intoxication varies by tetrahydrocannabinol (THC) level. However, cannabis testing does not assess THC levels in most US states, and testing rates among MVC decedents vary among states and over time, which may bias estimates of cannabis involvement. Researchers assessed cannabis involvement and THC levels among fatally injured drivers in Washington State before and after the legalization of non-medical (“recreational”) cannabis use, with and without imputation of missing cannabis testing data among the roughly half of decedents who were not tested.

• Using data from all MVC decedent drivers based on observed and imputed values, the prevalence of cannabis involvement in MVC fatalities was 9% prior to legalization and 19% after.
• In adjusted analyses, the proportion of decedent drivers with high THC levels (>10 ng/mL) increased nearly 5-fold after legalization.
• Although cannabis testing rates increased during the study period, findings were generally similar when restricted to those with completed cannabis testing.

Comments: This study is one of the first to impute cannabis involvement in MVC fatalities among decedents without testing, and to measure and impute THC levels (rather than simply the presence or absence of THC). Legalization of non-medical cannabis use in Washington State was associated with increases in cannabis involvement in MVC fatalities, including at levels clearly associated with impairment. These results add to literature suggesting that legalizing cannabis may increase MVC fatalities, and highlights the need to better characterize and mitigate those risks.

Timothy S. Naimi, MD, MPH

Reference: Tefft B, Arnold LS. Estimating cannabis involvement in fatal crashes in Washington State before and after recreational cannabis legalization using multiple imputation of missing values. Am J Epidemiol. 2021 [Epub ahead of print]. doi:10.1093/aje/kwab184