Recreational cannabis legalization caused increased traffic fatalities, primarily after retail sales began

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.

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.

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 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.

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.