Medical Marijuana Legalization Was Not Linked to More Tolerant Views of Cannabis-Impaired Driving

A 2024 study found no evidence that medical marijuana legalization made people more accepting of driving after using cannabis. The finding addresses a specific concern about normalization—not whether cannabis can impair driving or whether legalization affects crash rates.

Published in the Biometrical Journal, the study examined whether medical marijuana laws changed behaviors and attitudes related to driving after marijuana use, or DAMU. Researchers from Nationwide Children’s Hospital and The Ohio State University used a matched design intended to make observational survey data more useful for estimating a potential policy effect.

The analysis focused on Kentucky and Tennessee, neither of which had legalized medical marijuana during the study period. Respondents in those states were matched with similar respondents from Arkansas, Florida, Louisiana, Maryland, Michigan, New Jersey, Ohio and Pennsylvania—states that had legalized medical marijuana by 2017.

The researchers used data from the 2017 Traffic Safety Culture Index, an annual survey conducted by the AAA Foundation for Traffic Safety. The survey asked about four aspects of cannabis and driving:

• Whether respondents had driven within one hour of using marijuana during the previous year
• Whether they considered driving after marijuana use personally acceptable
• Whether they believed marijuana use had a positive, neutral or negative effect on driving
• Whether they viewed drug-impaired driving as a threat to their personal safety

These responses were combined into a score representing tolerance of DAMU. Contrary to the researchers’ hypothesis, the matched comparisons found no statistically persuasive evidence that medical marijuana legalization increased tolerance. The estimated effects were also in the opposite direction from what the normalization theory predicted, although the study was not designed to prove that legalization makes people less tolerant of impaired driving.

The results remained broadly similar in sensitivity analyses examining how strong an unmeasured confounding factor would need to be to change the conclusion. Even so, the authors did not present the findings as definitive proof that legalization has no effect on cannabis-related driving outcomes.

The study has several important limitations. It relied on self-reported behavior and opinions, which can be affected by recall and response bias. The analysis used a single year of survey data because 2017 was the only year in the dataset that included all four relevant questions. It also estimated a policy effect for people in Kentucky and Tennessee, so the results may not apply to every state or to later changes in cannabis policy.

Most importantly, the study measured reported behavior and attitudes—not driving performance, cannabis-related crash risk or the prevalence of impaired drivers on the road. Cannabis can impair reaction time, coordination, judgment and perception, and driving under the influence remains illegal. The Centers for Disease Control and Prevention’s guidance on cannabis and driving also notes that detecting THC does not by itself establish how impaired a particular driver is.

Other research has produced mixed results. Earlier analyses of Traffic Safety Culture Index data found some associations between medical marijuana laws and self-reported driving after marijuana use, while finding no consistent pattern that states with more liberal cannabis policies had more permissive attitudes. In Canada, official survey data showed that self-reported driving after cannabis use was stable in the first year after legalization, while later national data have reported declines from pre-legalization levels among some groups.

Taken together, the evidence does not support a simple claim that legalization automatically normalizes cannabis-impaired driving. But the absence of an observed change in attitudes in this study should not be interpreted as evidence that driving after cannabis use is safe. The findings instead suggest that legalization policy, public attitudes, actual cannabis use and traffic safety outcomes are separate questions that require different kinds of data.

As states continue to revise cannabis laws, researchers will need longer-term surveys, reliable measures of impairment and better crash data to determine how policy changes affect road safety. Until then, the practical safety message remains straightforward: people who have used cannabis or any other impairing substance should not drive.