Cannabis and traffic collision risk: findings from a case-crossover study of injured drivers presenting to emergency departments
DOI: 10.1007/s00038-013-0512-z
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Summary
This study investigates whether acute cannabis consumption increases the risk of traffic collisions, addressing inconsistencies in prior epidemiological literature regarding the magnitude and validity of this association. Previous research has suffered from methodological limitations, including the reliance on inactive THC metabolites that do not indicate acute impairment, the lack of appropriate control groups, and insufficient adjustment for confounding substances like ethanol. To resolve these issues, the authors employed a case-crossover design, which allows each participant to serve as their own control, thereby eliminating confounding from fixed personal characteristics such as age, driving experience, and personality. The study recruited 860 injured drivers presenting to emergency departments in Toronto and Halifax, Canada, between April 2009 and July 2011. Data collection involved both self-reported interviews and blood sample analysis. Acute cannabis use was defined by the presence of active THC metabolites (>0.2 ng/mL) in whole blood or via self-report if blood was unavailable. The case-crossover design compared substance use during the six hours preceding the collision against two control periods: the driver’s last similar driving episode and their usual frequency of driving under the influence over the preceding six months. Statistical analysis utilized conditional fixed effects logistic regression to assess collision risk while controlling for the presence of ethanol, benzodiazepines, and cocaine. Results indicated that 11.3% of drivers reported cannabis use prior to the collision, while 19.8% of those providing blood samples tested positive for THC. The primary analysis, combining blood and self-report data, found that cannabis use alone was associated with a four-fold increase in the odds of a collision (OR 4.11; 95% CI: 1.98–8.52). This risk increased further when cannabis was used in conjunction with other substances (OR 6.30). Sensitivity analyses revealed significant discrepancies between measurement methods: blood-only analysis showed a strong association (OR 12.0), whereas self-report-only analysis showed no significant association (OR 0.58). This divergence was attributed to underreporting, as many drivers with positive blood tests denied recent use. In contrast, ethanol consumption showed a consistent association with collision risk across all measurement methods. The findings confirm that acute cannabis consumption significantly increases traffic collision risk, reinforcing the need for policy and educational efforts aimed at reducing impaired driving. The study highlights the limitations of self-report data in this context, likely due to the illicit nature of cannabis and potential recall bias among chronic users. The authors conclude that objective biological measures, combined with rigorous study designs like case-crossover, are essential for accurately estimating the public health burden of cannabis-impaired driving. These results suggest that as cannabis use prevalence rises, particularly among young drivers, the associated collision risk remains a critical concern for road safety.
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| Stage | Outcome | Tool | Model | Prompt | Attempts | Completed |
|---|---|---|---|---|---|---|
| discover | success | OpenAlex-citations | — | — | 1 | 2026-06-20 |
| archive | success | unpaywall | — | — | 2 | 2026-06-26 |
| extract | success | pdftotext | — | — | 2 | 2026-06-26 |
| clean | success | clean | — | — | 1 | 2026-06-26 |
| chunk | success | chunk | — | — | 1 | 2026-06-26 |
| embed | success | embed | Qwen/Qwen3-Embedding-8B | — | 1 | 2026-06-26 |
| enrich | failed | — | — | — | 4 | 2026-06-26 |
| promote | success | — | — | — | 1 | 2026-06-20 |
| summarize | success | llm | qwen3.6-27b-prismaquant | summ-v5 | 1 | 2026-06-26 |
| tag | success | vector_similarity | — | — | 6 | 2026-06-26 |
| verify | success | — | — | — | 1 | 2026-06-26 |
Summary generated by qwen3.6-27b-prismaquant on 2026-06-26; verification: verified.
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- Empirical Findings: crash risk outcomes