Cannabis Legalization Linked to More Positive Drug Tests in Drivers but Mixed Effects on Crashes

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.

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.

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 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.

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.