A systematic review of 65 studies found cannabis legalization increased positive cannabis tests among drivers, while medical legalization was associated with fewer fatal crashes and recreational legalization with more.
People interested in how cannabis policy changes affect road safety
65 studies reviewed
What the researchers found
Medical legalization was associated with reductions in fatal motor vehicle collisions, while recreational legalization was associated with increases. Both forms of legalization increased cannabis-positive tests among drivers. Medical legalization was also associated with decreased positive alcohol tests among drivers.
Why it matters
As more jurisdictions legalize cannabis, understanding the road safety consequences is critical for policy decisions. The divergent findings for medical versus recreational legalization suggest different user populations may drive different outcomes.
The numbers in context
65 reports of 64 studies; 50 on recreational legalization; 22 on medical legalization; 5 on decriminalization; 39 used quasi-experimental designs; all but 1 used U.S. or Canadian data
How the study worked
Systematic review searching seven databases from inception to June 2021. Included 65 reports of 64 observational studies, of which 39 used quasi-experimental designs. Studies examined recreational legalization (n=50), medical legalization (n=22), and decriminalization (n=5). Nearly all studies used U.S. or Canadian data.
What this study cannot tell us
Most studies from U.S. and Canada, limiting global generalizability. Positive cannabis tests do not prove impairment at the time of driving. Very few studies examined decriminalization. Underlying study designs were observational, limiting causal conclusions.
How to read the evidence
Large systematic review with mostly quasi-experimental studies, but observational designs and heterogeneous outcomes limit certainty
When this study was published
2022 study
The bigger picture
The opposing findings for medical and recreational legalization may reflect different populations affected by each policy. Medical patients may substitute cannabis for alcohol or opioids (reducing impaired driving), while recreational access may add new impaired drivers to the road.
Questions still open
- Why do medical and recreational legalization have opposite associations with fatal crashes? Does the reduction in alcohol-positive tests with medical legalization reflect substitution? How does cannabis decriminalization affect road safety?
Common questions
Does cannabis legalization make roads more dangerous?
Does testing positive for cannabis mean a driver was impaired?
Read the original research
The Impact of Cannabis Decriminalization and Legalization on Road Safety Outcomes: A Systematic Review.
American journal of preventive medicine, 63(6), 1037-1052
Citation
Windle, Sarah B; Socha, Peter; Nazif-Munoz, José Ignacio; Harper, Sam; Nandi, Arijit. (2022). The Impact of Cannabis Decriminalization and Legalization on Road Safety Outcomes: A Systematic Review.. American journal of preventive medicine, 63(6), 1037-1052. https://doi.org/10.1016/j.amepre.2022.07.012