Effects of Legalization of Marijuana on Traffic: Transportation Research Synthesis
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Summary
This Transportation Research Synthesis (TRS) addresses the traffic safety implications of recreational marijuana legalization, motivated by Minnesota’s August 2023 legalization of recreational use. The study aims to inform Minnesota Department of Transportation (MnDOT) staff and policymakers by examining the experiences of other states. Specifically, it investigates changes in traffic safety outcomes, law enforcement challenges, and the guidelines and procedures used to determine driver impairment. The research methodology combined a comprehensive literature review with targeted surveys of state transportation and public safety agencies. The literature search covered domestic publications on traffic safety, public safety, law enforcement, and legal issues, focusing on marijuana policy, crash impacts, impairment detection, roadside testing devices, and Drug Recognition Expert (DRE) programs. Additionally, the researchers surveyed two groups of states: those with longer-term legalization (e.g., Alaska, Arizona, Colorado, Illinois, Michigan, New Jersey, Vermont) and those anticipating impacts from recent legalization (Delaware, Maryland, Missouri). The surveys gathered data on crash statistics, enforcement practices, training protocols, and lessons learned from 11 responding agencies. Findings from the literature indicate that while medical marijuana legalization was associated with reductions in fatal collisions, recreational legalization was conversely associated with increases in fatal crashes, though the extent varies and data quality remains a challenge. Research highlights that THC levels in oral fluid are not reliable indicators of intoxication and that confirmatory testing is required. Survey results from states with longer-term legalization revealed that five of seven responding agencies reported increases in fatal traffic crashes ranging from 16% to 22.3% since legalization. Conversely, two agencies reported decreases in injury crashes of 7% and 7.9%. Law enforcement respondents identified correlations between recreational use and speeding, alongside challenges such as drivers refusing testing, lack of updated laws, and adverse court rulings regarding per se THC limits. All surveyed states identified a need for more DREs, with staffing levels varying widely from 37 to 546 officers. States anticipating recent legalization impacts generally expect increases in serious injuries and fatalities and are implementing roadside testing pilots using oral fluid devices like SoToxa. The significance of this synthesis lies in its provision of actionable strategies for agencies managing post-legalization traffic safety. Recommended measures include deploying speed enforcement details, enhancing forensic practices, improving public awareness, and increasing law enforcement training. The report underscores the critical role of DREs as expert witnesses and the necessity of robust training programs, such as the Drug Evaluation and Classification Program (DECP) and Advanced Roadside Impaired Driving Enforcement (ARIDE). By documenting the operational realities and crash data trends in other jurisdictions, the TRS provides MnDOT and legislators with evidence-based insights to assess risks and refine enforcement policies as Minnesota implements its new recreational marijuana laws.
Key finding
The legalization of recreational marijuana is associated with mixed traffic safety outcomes, including increases in fatal crashes in some states and decreases in injury crashes in others, alongside significant enforcement challenges related to impairment detection and testing reliability.
Methodology
mixed_methods
Sample size: 11
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 |
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| 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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- Applied Guidance: countermeasure evaluation, policy recommendations
- Empirical Findings: observational prevalence