Recreational cannabis legalization in US states was associated with a 15% increase in fatal motor vehicle collisions and a 16% increase in associated deaths, with no conclusive difference between the first year and later years.
Transportation safety researchers, policymakers, law enforcement, and public health officials.
15% increase in fatal motor vehicle collisions after cannabis legalization
What the researchers found
After adjusting for calendar year, legalization was associated with increased rates of fatal collisions (IRR 1.15, 95% CI 1.06-1.26) and associated deaths (IRR 1.16, 95% CI 1.06-1.27). The difference between the first 12 months post-legalization and subsequent months was inconclusive.
Why it matters
This multi-state analysis provides population-level evidence of increased traffic fatalities associated with cannabis legalization, informing ongoing policy debates about legalization and road safety.
The numbers in context
Fatal collision IRR: 1.15 (95% CI 1.06-1.26). Death IRR: 1.16 (95% CI 1.06-1.27). Data period: 2007-2018. First 12 months vs. later: IRR 0.92 for collisions and deaths (not significant).
How the study worked
Ecologic study using the US Fatality Analysis Reporting System (2007-2018). Examined jurisdiction-specific fatal collision rates before and after legalization using Poisson regression, then meta-analyzed across jurisdictions using DerSimonian and Laird random-effects models.
What this study cannot tell us
Ecologic design cannot link individual cannabis use to specific crashes. Cannot control for all confounding factors that changed alongside legalization. US Fatality Analysis Reporting System may have reporting variations across jurisdictions.
How to read the evidence
Multi-state analysis with meta-analytic approach, but ecologic design limits causal inference.
When this study was published
Published in 2021 using US data from 2007-2018.
The bigger picture
While the 15% relative increase is statistically significant, it raises important questions about whether improved enforcement, public education, or impaired driving detection could mitigate these effects.
Questions still open
- Are the increased fatalities directly caused by cannabis-impaired driving, or do other behavioral changes accompanying legalization play a role? Would improved impairment detection reduce this effect?
Common questions
Did fatal car crashes increase after cannabis legalization?
Did the increase get worse over time?
Read the original research
Association between legalization of recreational cannabis and fatal motor vehicle collisions in the United States: an ecologic study.
CMAJ open, 9(1), E233-E241
Citation
Windle, Sarah B; Eisenberg, Mark J; Reynier, Pauline; Cabaussel, Josselin; Thombs, Brett D; Grad, Roland; Ells, Carolyn; Sequeira, Crystal; Filion, Kristian B. (2021). Association between legalization of recreational cannabis and fatal motor vehicle collisions in the United States: an ecologic study.. CMAJ open, 9(1), E233-E241. https://doi.org/10.9778/cmajo.20200155