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.
Read this if you want the evidence on how cannabis affects driving ability and crash risk.
Cannabis alone: 2-3x crash risk. Cannabis + alcohol: 11x injury risk
What the researchers 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.
Why it 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 numbers in context
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 the study worked
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.
What this study cannot 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.
How to read the evidence
Comprehensive narrative review synthesizing 36 studies from multiple databases. Strong convergent evidence from experimental and epidemiological sources.
When this study was published
Published in 2017. Cannabis and driving research has continued with more refined dose-response and real-world driving studies.
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.
Questions still open
- 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?
Common questions
How does cannabis impair driving differently from alcohol?
How long should you wait to drive after using cannabis?
Read the original research
Neurocognitive Correlates in Driving Under the Influence of Cannabis.
CNS & neurological disorders drug targets, 16(5), 534-540
Citation
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