Importance: The impact of nonmedical cannabis legalization on traffic injuries and cannabis involvement in traffic injuries is unclear.

Question: Have cannabis-involved traffic injury emergency department visits changed after cannabis legalization and the subsequent commercialization of the cannabis retail market (ie, store and product expansion) in Ontario, Canada?

Findings: In this cross-sectional study capturing 426 cannabis-involved traffic injury emergency department visits, annual rates of cannabis involvement increased by 475.3% over 13 years. After accounting for time trends, legalization with restrictions was not associated with increased cannabis involvement during traffic injury emergency department visits; however, market commercialization, which overlapped with the COVID-19 pandemic, was.

Meaning: These findings suggest that cannabis-involved traffic injuries have increased over time and that the commercialization of cannabis markets may result in further increases.