THC-Positive Drivers in Fatal Crashes Doubled After Washington Legalized Cannabis
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.
Quick Facts
What This Study 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).
Key Numbers
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 They Did This
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.
Why This Research 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 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.
What This Study Doesn't 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.
Questions This Raises
- ?Would enhanced impaired driving enforcement reduce the post-legalization increase?
- ?Are states that legalized after Washington implementing more effective driving-while-high prevention?
Trust & Context
- Key Stat:
- Drivers with high THC concentrations in fatal crashes increased nearly five-fold after legalization
- Evidence Grade:
- Large population-based analysis with innovative imputation approach, though limited by observational design and inability to prove THC caused crashes.
- Study Age:
- Data from 2008-2019 in Washington state.
- Original Title:
- Estimating Cannabis Involvement in Fatal Crashes in Washington State Before and After the Legalization of Recreational Cannabis Consumption Using Multiple Imputation of Missing Values.
- Published In:
- American journal of epidemiology, 190(12), 2582-2591 (2021)
- Authors:
- Tefft, Brian C, Arnold, Lindsay S
- Database ID:
- RTHC-03572
Evidence Hierarchy
Looks back at existing records to find patterns.
What do these levels mean? →Frequently Asked Questions
Did fatal crashes increase after cannabis legalization?
This study found the proportion of THC-positive drivers in fatal crashes more than doubled after legalization in Washington state. However, THC presence doesn't prove impairment caused the crash, and overall crash trends involve many factors.
Why is the increase in high-concentration THC drivers important?
The nearly five-fold increase in drivers with high THC levels (as opposed to just trace amounts) suggests more people are driving shortly after using cannabis, not merely that detection rates improved.
Read More on RethinkTHC
Cite This Study
https://rethinkthc.com/research/RTHC-03572APA
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
MLA
Tefft, Brian C, et al. "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, 2021. https://doi.org/10.1093/aje/kwab184
RethinkTHC
RethinkTHC Research Database. "Estimating Cannabis Involvement in Fatal Crashes in Washingt..." RTHC-03572. Retrieved from https://rethinkthc.com/research/tefft-2021-estimating-cannabis-involvement-in
Access the Original Study
Study data sourced from PubMed, a service of the U.S. National Library of Medicine, National Institutes of Health.
This study breakdown was produced by the RethinkTHC research team. We analyze and report published research findings without making health recommendations. All interpretations are based solely on the published abstract and study data.