Using augmented synthetic control methods across US states, researchers found a consistent but lagged increase in traffic fatality rates after recreational cannabis legalization, primarily linked to retail availability.
Traffic safety researchers, cannabis policy designers, state legislators considering legalization
Retail availability, not legal status, drove fatality increases
What the researchers found
Using an augmented synthetic control method to generate causal inference, the study identified a consistent but lagged pattern of increased traffic fatality rates in several states following recreational marijuana legalization. The effect was primarily linked to the beginning of retail sales rather than the legalization date itself. The findings disprove conjectures dismissing the link between recreational marijuana use and fatal traffic crashes.
Why it matters
This study uses one of the most rigorous causal inference methods available for policy evaluation and finds that recreational cannabis legalization does increase traffic fatalities, but only after retail sales begin. This distinction is critical for policy design: the transition to commercial availability, not legal status alone, drives the safety impact.
The numbers in context
Multiple US states analyzed; consistent lagged increase in fatality rates post-legalization; effect primarily linked to retail availability rather than legalization date
How the study worked
Augmented synthetic control method applied to US state-level traffic fatality data. This approach creates synthetic comparison states from non-legalizing states to estimate what would have happened without legalization, adjusting for unobserved confounders. Multiple states analyzed across different legalization timelines.
What this study cannot tell us
Augmented synthetic control cannot fully account for all confounding changes that coincide with legalization. Traffic fatality data may not capture the full picture of impaired driving incidents. Cannot distinguish between cannabis-impaired driving and other behavioral changes associated with legalization.
How to read the evidence
Augmented synthetic control method provides strong causal inference for policy effects, though residual confounding and inability to measure impairment directly are limitations.
When this study was published
2025 publication
The bigger picture
The debate over whether cannabis legalization increases traffic fatalities has been contentious, with mixed findings from simpler analytical approaches. This study's use of a more rigorous causal method provides stronger evidence for the link while pinpointing retail availability as the key mechanism.
Questions still open
- Would delaying retail sales after legalization reduce the traffic safety impact? Do states with stricter impaired driving laws for cannabis see smaller increases?
Common questions
Does legalizing cannabis increase traffic deaths?
What makes this study different from previous research?
Read the original research
Evaluation of the causal impact of recreational marijuana legalisation on traffic safety in the US.
Accident; analysis and prevention, 220, 108106
Citation
Anupriya; McCoy, Emma; Graham, Daniel J. (2025). Evaluation of the causal impact of recreational marijuana legalisation on traffic safety in the US.. Accident; analysis and prevention, 220, 108106. https://doi.org/10.1016/j.aap.2025.108106