Prevalence of Marijuana Involvement in Fatal Crashes: Washington, 2010-2014
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Summary
This study quantifies the prevalence of marijuana involvement in fatal motor vehicle crashes in Washington State between 2010 and 2014, specifically investigating changes following the enactment of Initiative 502, which legalized recreational marijuana use for adults and established a per se limit for driving under the influence. The research was motivated by the need to understand the impact of legalization on traffic safety, as prior data limitations had obscured the prevalence of recent marijuana use among crash-involved drivers. The researchers analyzed a census of 3,031 drivers involved in fatal crashes using data from the Washington Traffic Safety Commission. Because less than half of the drivers had usable blood toxicology results for delta-9-tetrahydrocannabinol (THC), the study employed multiple imputation to estimate THC presence and concentration for untested drivers. This method accounted for variables associated with the probability of testing and testing positive, such as crash circumstances and driver demographics. The analysis focused on THC concentrations in blood, adjusting for changes in laboratory detection thresholds during the study period. Overall, an estimated 10.0% of drivers involved in fatal crashes had detectable THC in their blood. The prevalence remained relatively stable from 2010 to 2013, ranging from 7.9% to 8.5%, but approximately doubled in 2014 to 17.0%. Trend analysis indicated that the proportion of THC-positive drivers was flat before and immediately after Initiative 502 took effect in December 2012. However, beginning approximately nine months later, in September 2013, the prevalence began increasing significantly at a rate of 9.7 percentage points per year. Among THC-positive drivers, 34.0% had no other substances in their blood, while 39.0% also had detectable alcohol. Drivers who died in the crash were more likely to have higher THC concentrations and to have used alcohol or other drugs concurrently compared to surviving drivers. The findings suggest a delayed increase in marijuana involvement in fatal crashes following legalization, though the study notes that the specific cause of this trend—whether attributable to Initiative 502 or other factors—remains unclear. The results highlight the importance of using detailed toxicology data and statistical imputation to accurately assess drug involvement in traffic fatalities, as standard reporting methods often underestimate prevalence. The study concludes that while detectable THC is present in a significant minority of fatal crashes, the data cannot determine impairment or fault, underscoring the complexity of linking marijuana use to crash causation.
Key finding
An estimated 10.0% of Washington fatal-crash-involved drivers (2010–2014) had detectable THC; annual prevalence approximately doubled from 8.3% in 2013 to 17.0% in 2014, with piecewise trend analysis indicating a significant increase of 9.7 percentage points per year beginning roughly nine months after Initiative 502 took effect, though causation to the law versus other factors could not be established.
Methodology
modeling
Sample size: 3,031 drivers in 2,070 fatal crashes (Washington State FARS, 2010–2014)
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