Review of 36 studies confirms cannabis impairs driving skills and doubles or triples crash risk
A comprehensive review of experimental, simulator, and epidemiological studies found that cannabis impairs critical driving functions and doubles or triples crash risk, with combined alcohol-cannabis exposure increasing injury risk nearly 11-fold.
Quick Facts
What This Study Found
The review synthesized findings from 36 studies on cannabis and driving. Both experimental and epidemiological evidence converged on clear impairment. Cannabis affected critical-tracking tasks, increased lane weaving, decreased reaction time, and impaired divided attention.
Driving under the influence of cannabis alone doubled or tripled crash risk. When cannabis was combined with alcohol, the odds ratio for driving-related injury jumped to 10.9, compared to 5.8 for cannabis alone in a case-crossover study.
Brain imaging studies revealed both acute and chronic structural and functional changes in cognitive regions relevant to driving, including attention, executive function, and psychomotor control.
Key Numbers
Cannabis alone: 2-3x crash risk. Cannabis + alcohol: OR 10.9 for driving-related injury vs 5.8 for cannabis alone. 36 studies reviewed across multiple databases. Impairments: critical tracking, lane weaving, reaction time, divided attention.
How They Did This
Review of publications from PubMed, Cochrane Central, Scopus, Web of Science, Science Direct, EMBASE, and Google Scholar through January 2017. Selected 36 publications covering experimental studies (simulators, on-road tests), brain imaging, and epidemiological crash risk data.
Why This Research Matters
As cannabis use becomes more normalized, evidence-based information about driving impairment is critical. This review provides a comprehensive picture showing convergent evidence from multiple study types that cannabis meaningfully impairs driving, and that the combination with alcohol is particularly dangerous.
The Bigger Picture
The evidence against cannabis-impaired driving is now comparable in quality to the evidence against alcohol-impaired driving, though cannabis impairment manifests differently (more cautious but less attentive, versus more reckless with alcohol). The 11-fold injury risk from combining both substances underscores that polysubstance use before driving is an urgent public health issue.
What This Study Doesn't Tell Us
The review is not systematic (no PRISMA methodology). Included studies used varying THC doses, administration routes, and impairment assessments. Epidemiological studies face the challenge that THC remains detectable long after impairment has resolved. The 2-3x crash risk estimate is an aggregate that may vary by dose, tolerance, and individual factors.
Questions This Raises
- ?What THC blood level best predicts driving impairment?
- ?How long after cannabis use is it safe to drive?
- ?Do regular users develop enough tolerance that their driving impairment is reduced?
- ?How should drugged driving laws account for the disconnect between THC detection and actual impairment?
Trust & Context
- Key Stat:
- Cannabis alone: 2-3x crash risk. Cannabis + alcohol: 11x injury risk
- Evidence Grade:
- Comprehensive narrative review synthesizing 36 studies from multiple databases. Strong convergent evidence from experimental and epidemiological sources.
- Study Age:
- Published in 2017. Cannabis and driving research has continued with more refined dose-response and real-world driving studies.
- Original Title:
- Neurocognitive Correlates in Driving Under the Influence of Cannabis.
- Published In:
- CNS & neurological disorders drug targets, 16(5), 534-540 (2017)
- Authors:
- Busardò, Francesco P(2), Pellegrini, Manuela, Klein, Julia, di Luca, Natale M
- Database ID:
- RTHC-01347
Evidence Hierarchy
Summarizes existing research on a topic.
What do these levels mean? →Frequently Asked Questions
How does cannabis impair driving differently from alcohol?
Cannabis primarily impairs attention, reaction time, lane tracking, and divided attention. Alcohol tends to make drivers more reckless, while cannabis users often drive more cautiously but with reduced awareness and slower reactions. However, the combined effect of both substances is far worse than either alone.
How long should you wait to drive after using cannabis?
This review did not establish a specific safe waiting period. THC impairment typically peaks within the first 1-2 hours after smoking but can last longer with edibles or higher doses. The disconnect between THC blood levels and actual impairment makes it difficult to set a precise timeline.
Read More on RethinkTHC
Cite This Study
https://rethinkthc.com/research/RTHC-01347APA
Busardò, Francesco P; Pellegrini, Manuela; Klein, Julia; di Luca, Natale M. (2017). Neurocognitive Correlates in Driving Under the Influence of Cannabis.. CNS & neurological disorders drug targets, 16(5), 534-540. https://doi.org/10.2174/1871527316666170424115455
MLA
Busardò, Francesco P, et al. "Neurocognitive Correlates in Driving Under the Influence of Cannabis.." CNS & neurological disorders drug targets, 2017. https://doi.org/10.2174/1871527316666170424115455
RethinkTHC
RethinkTHC Research Database. "Neurocognitive Correlates in Driving Under the Influence of ..." RTHC-01347. Retrieved from https://rethinkthc.com/research/busardo-2017-neurocognitive-correlates-in-driving
Access the Original Study
Study data sourced from PubMed, a service of the U.S. National Library of Medicine, National Institutes of Health.
This study breakdown was produced by the RethinkTHC research team. We analyze and report published research findings without making health recommendations. All interpretations are based solely on the published abstract and study data.