Marijuana, Other Drugs, and Alcohol Use by Drivers in Washington State
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Summary
This study investigates the prevalence of alcohol and drug use among drivers in Washington State following the legalization of recreational marijuana sales. Motivated by public safety concerns regarding the impact of legalized cannabis on traffic safety, the research aimed to assess changes in drugged driving before and after the implementation of legal retail sales, which began on July 8, 2014. The study specifically focused on detecting delta-9-tetrahydrocannabinol (THC), its active metabolite, and other prescription, over-the-counter, and illegal substances. The researchers conducted a voluntary, anonymous roadside survey across six counties in Washington State. Data collection occurred in three distinct waves: immediately before legal sales (Wave 1), approximately six months after implementation (Wave 2), and one year after implementation (Wave 3). Each wave included observational data and biological samples collected during specific daytime and nighttime sessions. The study analyzed breath samples from 2,423 drivers for alcohol, oral fluid samples from 2,313 drivers, and blood samples from 1,929 drivers for drugs. Drug detection involved a two-stage process: initial screening followed by confirmatory testing for positive results. Statistical analyses compared Wave 1 to subsequent waves, with significance set at p < .05. The results indicated an upward trend in THC-positive drivers across the three waves, rising from 14.6% in Wave 1 to 19.4% in Wave 2 and 21.4% in Wave 3. However, these overall increases were not statistically significant due to high variability between sites. Notably, there was a statistically significant increase in the prevalence of THC-positive drivers during daytime hours, rising from 7.8% in Wave 1 to 18.4% in Wave 2 and 18.9% in Wave 3. Nighttime prevalence also increased but did not reach statistical significance. Male drivers showed significantly higher THC prevalence in Wave 3 compared to Wave 1. Alcohol-positive drivers remained low, ranging from 3.9% to 6.0% across waves, with no significant changes. Additionally, the proportion of drivers testing positive for THC but negative for alcohol increased significantly, while the prevalence of illegal drugs (excluding THC) declined from 2.4% to 0.1%. The study concludes that while the overall increase in THC-positive drivers was not statistically significant, the significant rise in daytime THC prevalence suggests a shift in usage patterns following legalization. Furthermore, all three waves of the Washington State study showed significantly higher rates of THC-positive drivers compared to the 2013–2014 National Roadside Survey. The findings imply that legal access to marijuana may have increased the presence of THC in drivers, particularly during daytime hours, highlighting the need for continued monitoring of impaired driving trends in jurisdictions with legalized cannabis.
Key finding
Daytime THC-positive driver prevalence increased significantly from 7.8 percent before legalization to 18.4 percent and 18.9 percent in subsequent waves, while overall THC-positive rates rose from 14.6 percent to 21.4 percent without reaching statistical significance.
Methodology
field_study
Sample size: 2400
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_rosap on 2026-05-23 (6 acquisition events logged).
| Stage | Outcome | Tool | Model | Prompt | Attempts | Completed |
|---|---|---|---|---|---|---|
| discover | success | rosap | — | — | 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 | 3 | 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: observational prevalence