The effects of recreational marijuana legalization on driving under the influence of marijuana in a neighboring state: A cross-border quasi-experiment

• Even if a state has not legalized recreational marijuana, recreational marijuana legalization in bordering states may impact traffic safety, as measured by crashes involving marijuana-positive drivers. • This study used a quasi-experimental stepped wedge design that leveraged temporal and spatial variation in recreational marijuana legalization in states that border Pennsylvania. • On average, a Pennsylvania county experienced a significant 58.41% increase in Pennsylvania State Police-reported crashes involving a marijuana-positive driver once the county began to border a recreational marijuana state. Once it began to border two states with recreational marijuana legalization, a Pennsylvania county experienced, on average, a nonsignificant 55.48% increase. • Future research should quantitatively assess mechanisms for this association as well as actionable and evidence-based strategies to prevent driving under the influence of marijuana. Objectives : This study examines whether recreational marijuana (RM) legalization in neighboring states is associated with driving under the influence of marijuana in a non-legalizing state, as measured by motor-vehicle crashes involving a marijuana-positive driver (MP). Methods : Although Pennsylvania has not legalized RM, it borders five states that legalized RM at different time points in 2021 and 2023. This spatial and temporal variation enables a quasi-experimental stepped wedge design that assesses whether Pennsylvania counties that border a state with RM legalization experienced changes in MP motor-vehicle crash counts. Pennsylvania Department of Transportation data provide crash counts at the county-month level (January 2019–December 2024). Of primary interest are MP crashes where the Pennsylvania State Police (PSP) was the reporting law enforcement agency (MP-PSP). Once RM legalization occurs in a contiguous state, a Pennsylvania county (N=27) bordering that state becomes Post-Exposure. Four Pennsylvania counties eventually bordered two states with RM legalization, becoming Post-Second-Exposure. Poisson regression was used to estimate changes in crashes in the Post-Exposure and Post-Second-Exposure periods relative to the Pre-Exposure period. Results: On average, a Pennsylvania county experienced a significant 58.41% increase in MP-PSP crashes Post-Exposure and a nonsignificant 55.48% increase Post-Second-Exposure. Where a border county has an interstate highway (N=20), on average, there was a significant 68.95% increase Post-Exposure and a nonsignificant 56.80% increase Post-Second-Exposure. Practical Applications: RM legalization in bordering states may impact traffic safety in a state that has not yet legalized RM. Future research should identify mechanisms for this association as well as actionable strategies to prevent marijuana-impaired driving.