After Washington state legalized recreational cannabis, the proportion of THC-positive drivers in fatal crashes more than doubled (9.3% to 19.1%), with drivers having high THC concentrations increasing nearly five-fold.
Traffic safety researchers, cannabis policy legislators, public health officials
Drivers with high THC concentrations in fatal crashes increased nearly five-fold after legalization
What the researchers found
Using multiple imputation to account for untested drivers, the proportion of THC-positive drivers in fatal crashes rose from 9.3% before to 19.1% after legalization (adjusted PR=2.3, 95% CI: 1.3-4.1). Drivers with high THC concentrations increased nearly five-fold (adjusted PR=4.7, 95% CI: 1.5-15.1).
Why it matters
The sharp increase in high-THC drivers after legalization suggests more people are driving shortly after cannabis use, not just that more people have detectable THC from prior use.
The numbers in context
8,282 drivers in fatal crashes; 2008-2019; nearly half untested (addressed by imputation); THC-positive: 9.3% before vs 19.1% after legalization; adjusted PR=2.3; high THC concentrations: adjusted PR=4.7.
How the study worked
Retrospective analysis of all 8,282 drivers involved in fatal crashes in Washington state (2008-2019), using multiple imputation for the nearly half of drivers not tested for drugs, with logistic regression and marginal standardization.
What this study cannot tell us
Multiple imputation introduces uncertainty; THC presence does not prove impairment; no causal inference possible; Washington-specific results may not generalize; cannot distinguish medical from recreational use.
How to read the evidence
Large population-based analysis with innovative imputation approach, though limited by observational design and inability to prove THC caused crashes.
When this study was published
Data from 2008-2019 in Washington state.
The bigger picture
These findings demonstrate the importance of incorporating impaired driving countermeasures into cannabis legalization frameworks, as the relationship between legalization and traffic safety is not simply a matter of detecting more users.
Questions still open
- Would enhanced impaired driving enforcement reduce the post-legalization increase? Are states that legalized after Washington implementing more effective driving-while-high prevention?
Common questions
Did fatal crashes increase after cannabis legalization?
Why is the increase in high-concentration THC drivers important?
Read the original research
Estimating Cannabis Involvement in Fatal Crashes in Washington State Before and After the Legalization of Recreational Cannabis Consumption Using Multiple Imputation of Missing Values.
American journal of epidemiology, 190(12), 2582-2591
Citation
Tefft, Brian C; Arnold, Lindsay S. (2021). Estimating Cannabis Involvement in Fatal Crashes in Washington State Before and After the Legalization of Recreational Cannabis Consumption Using Multiple Imputation of Missing Values.. American journal of epidemiology, 190(12), 2582-2591. https://doi.org/10.1093/aje/kwab184