Cannabis Use Among Drivers in Fatal Crashes in Washington State Before and After Legalization
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Summary
This research brief by the AAA Foundation for Traffic Safety examines the prevalence of cannabis use among drivers involved in fatal crashes in Washington State before and after the legalization of recreational cannabis. The study was motivated by Initiative 502 (I-502), which took effect in December 2012, legalizing possession for adults aged 21 and older while establishing a per se limit of 5 nanograms of delta-9-tetrahydrocannabinol (THC) per milliliter of blood for drivers. A previous 2016 study indicated that the proportion of THC-positive drivers in fatal crashes had approximately doubled shortly after legalization. This 2020 report updates that analysis with three additional years of data to determine whether this increase was temporary, stabilized, or continued to rise. The study analyzed data from 6,721 drivers involved in fatal crashes in Washington State between 2008 and 2017, obtained from the Washington Traffic Safety Commission. The dataset included detailed toxicology reports specifying whether drivers were tested, the specimen type (blood or urine), and drug results. Drivers were classified as THC-positive if their blood tested positive for THC, THC-negative if blood tested negative or urine tested negative for cannabinoids, and THC-unknown if they were not tested, only had urine tested, or had results obscured by reporting limits. To address missing data, the researchers employed multiple imputation to estimate the THC status of drivers who were not tested or had unavailable results. The imputation model accounted for linear trends, pre- and post-legalization indicators, and demographic factors such as age, sex, and urban/rural location. The results indicate that the proportion of fatal-crash-involved drivers who were THC-positive remained approximately double the pre-legalization level five years after I-502 took effect. Before legalization, an average of 8.8% of drivers in fatal crashes were estimated to be THC-positive. After legalization, this average increased to 18.0%, with the increase occurring gradually throughout 2013 and stabilizing in 2014. The estimated proportion reached its highest point in the study period in 2017, at 21.4%. The raw number of drivers testing positive for THC more than tripled over the decade, and the total estimated number of THC-positive drivers nearly tripled. While data from 2015 to 2017 suggested a slight continuing upward trend, the authors noted uncertainty regarding whether this reflected a true increase or random fluctuation. The significance of these findings lies in the sustained association between recreational cannabis legalization and increased THC presence in fatal crashes. However, the authors emphasize that detectable THC in blood does not conclusively prove impairment at the time of the crash, as THC levels decline rapidly after smoking and can remain detectable in frequent users for days or weeks. Consequently, while the prevalence of THC-positive drivers has doubled and stabilized at that higher level, the study does not establish causality regarding crash fault or impairment. The authors conclude that more research is needed to understand the specific role of cannabis in motor vehicle fatalities and the impact of related regulations.
Key finding
Five years after I-502, the estimated proportion of Washington fatal-crash-involved drivers who were THC-positive remained about twice the pre-legalization level, reaching 21% in 2017—the highest share in the 2008–2017 study period.
Methodology
modeling
Sample size: n=6721 drivers involved in fatal crashes (Washington State, 2008-2017)
Provenance
The full processing record for this entry. Every stage of this paper's journey through the pipeline is logged — what ran, with which tool and model, how many attempts it took, and when it last completed. Discovered via bulk_ingest_aaa_foundation on 2026-05-23 (5 acquisition events logged).
| Stage | Outcome | Tool | Model | Prompt | Attempts | Completed |
|---|---|---|---|---|---|---|
| discover | success | aaa_foundation | — | — | 2 | 2026-05-23 |
| archive | success | — | — | — | 1 | 2026-05-23 |
| extract | success | cached | — | — | 2 | 2026-06-10 |
| clean | success | — | — | — | 1 | 2026-06-01 |
| chunk | success | — | — | — | 1 | 2026-06-01 |
| embed | success | — | — | — | 1 | 2026-06-02 |
| enrich | success | — | — | — | 1 | 2026-05-23 |
| promote | success | — | — | — | 1 | 2026-05-23 |
| summarize | success | llm | qwen3.6-27b-prismaquant | summ-v5 | 2 | 2026-06-10 |
| tag | success | vector_similarity | — | — | 19 | 2026-06-11 |
| verify | success | — | — | — | 2 | 2026-06-10 |
Summary generated by qwen3.6-27b-prismaquant on 2026-06-10; verification: verified.
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- Empirical Findings: crash risk outcomes