Cannabis, Impaired Driving, and Road Safety: An Overview of Key Questions and Issues

Brands, Bruna; Di Ciano, Patricia; Mann, Robert E. · 2021 · Crossref

DOI: 10.3389/fpsyt.2021.641549

archive: archived pipeline: cataloged verified

Get this paper ↗ (DOI — opens at the source; we link to it, we don't host it)

Summary

This review paper examines the impact of cannabis on road safety, addressing key questions regarding collision risk, behavioral effects, and pharmacological factors. The research was motivated by the legalization of non-medical cannabis in jurisdictions like Canada and Uruguay, which heightened concerns about driving under the influence of cannabis (DUIC). The authors synthesize existing literature from epidemiological, pharmacological, and behavioral sciences to evaluate whether cannabis increases collision risk, how it affects driving behavior, and what implications this has for policy and public health. The paper reviews epidemiological studies, laboratory simulator studies, and observational data. Epidemiological evidence indicates that acute cannabis use increases collision risk, with meta-analyses confirming this association. Laboratory studies utilize driving simulators to assess the effects of different administration routes (smoking, vaping, oral), doses, and user types (medical vs. recreational, frequent vs. occasional). Specific metrics analyzed include Standard Deviation of Lateral Position (SDLP), speed, reaction time, and headway. The review also considers sex differences, tolerance development, and the relationship between blood THC levels and impairment. Key findings indicate that cannabis use, particularly smoking, consistently increases SDLP (weaving) and decreases driving speed, though effects on reaction time and lane position are mixed. Vaping shows variable effects depending on task parameters, while oral administration (studied via synthetic THC) increases SDLP and reaction time. The review finds no evidence that medical users develop tolerance to driving impairments; medical users exhibit similar speed reductions and THC blood levels as recreational users. Frequent users show mixed results regarding tolerance, with some studies suggesting occasional users are more impaired than regular users. A dose-response relationship exists, with impairments observed at low THC levels (e.g., 2 ng/ml). CBD does not appear to mitigate THC-induced driving deficits. Impairment typically lasts only a few hours post-consumption, declining as blood THC levels drop below 3–5 ng/ml, despite THC metabolites remaining detectable for longer periods. The significance of these findings lies in informing road safety policies and legal limits. The authors conclude that cannabis impairs driving regardless of medical or recreational use, challenging claims of tolerance among medical users. The poor correlation between biological THC levels and impairment degree complicates the establishment of "per se" legal limits, though proposed equivalents to alcohol limits range from 3–10 ng/ml. The review highlights gaps in knowledge regarding collision characteristics, injury severity, and the specific effects of edibles and CBD combinations, calling for further research to refine safety guidelines and regulatory frameworks.

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.

Summary generated by qwen3.6-27b-prismaquant on 2026-06-26; verification: verified.

Topics

Ranked by relevance to this paper. Hover a topic for its definition.

• cannabis
• polydrug
• illicit drugs