Drivers’ Use of Marijuana in Washington State [Traffic Tech]
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Summary
This study, conducted by the National Highway Traffic Safety Administration (NHTSA) in collaboration with the Washington Traffic Safety Commission, examines the prevalence of marijuana use among drivers in Washington State following the legalization of recreational marijuana sales in July 2014. The primary objective was to determine if the percentage of drivers testing positive for marijuana increased after legalization and to assess any effects on alcohol-impaired driving. The research design involved three data collection waves: immediately before legal sales began, six months after implementation, and one year after implementation. Data were collected from nearly 2,400 participants across six sites in Washington State. Drivers were randomly selected at various locations within these sites during specific time windows: Friday afternoons and Friday/Saturday nights. To ensure safety and anonymity, police officers were present, and drivers were offered monetary incentives ($10 for oral fluid, $50 for blood samples). Alcohol use was determined via breath samples, while drug use was assessed through oral fluid or blood samples. The study screened for over 70 substances, including over-the-counter, prescription, and illegal drugs. A key methodological limitation is that the study measured the prevalence of THC (delta-9-tetrahydrocannabinol) presence rather than acute impairment, as THC can remain detectable for days or weeks after use. Additionally, the baseline data may be inflated because legal sales had already begun by the time the first wave was conducted. The results indicated that marijuana and alcohol were the most common substances detected. Across the three waves, 14.6%, 19.4%, and 21.4% of drivers tested positive for marijuana, respectively; however, these increases were not statistically significant overall. Notably, there was a statistically significant increase in the daytime prevalence of marijuana-positive drivers, rising from 7.8% in Wave 1 to 18.9% in Wave 3. Nighttime prevalence also increased (from 17.5% to 22.2%) but did not reach statistical significance. Alcohol use remained stable, with 6.0%, 3.9%, and 4.4% of participants testing positive across the waves. Concurrent use of alcohol and marijuana was rare, ranging from 0.04% to 1.3%. Regarding Washington’s per se limit of 5 ng/mL of THC, the percentage of drivers exceeding this limit dropped significantly from 14.5% in Wave 1 to 5.3% in Wave 2, before rising to 9.2% in Wave 3. The study concludes that while marijuana prevalence among drivers increased, particularly during daytime hours, the overall changes were not statistically significant. The findings suggest that legalization did not lead to a significant increase in alcohol-impaired driving or combined alcohol-marijuana use. However, the authors emphasize that detecting THC presence does not equate to determining impairment or crash risk. The data serves as a tool for tracking trends in drug-positive drivers to inform traffic safety efforts, though generalizability is limited by the specific timing, location, and sampling methods used.
Key finding
Across three roadside survey waves, drivers testing positive for marijuana rose from 14.6 to 21.4 percent, with a statistically significant daytime increase from 7.8 to 18.9 percent, while alcohol-positive rates did not change significantly.
Methodology
field_study
Provenance
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| Stage | Outcome | Tool | Model | Prompt | Attempts | Completed |
|---|---|---|---|---|---|---|
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| extract | success | cached | — | — | 2 | 2026-06-10 |
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| summarize | success | llm | qwen3.6-27b-prismaquant | summ-v5 | 3 | 2026-06-10 |
| tag | success | vector_similarity | — | — | 19 | 2026-06-11 |
| verify | success | — | — | — | 3 | 2026-06-10 |
Summary generated by qwen3.6-27b-prismaquant on 2026-06-10; verification: verified.
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- Empirical Findings: observational prevalence