Extreme heat disproportionately increases severe road traffic crashes in high conflict settings and among vulnerable road users in California

Emerging evidence suggests that extreme heat elevates road-traffic injuries, undermining international road safety efforts like Vision Zero as global warming intensifies. However, the mechanisms underlying heat-related crashes remain poorly understood, with limited research linking heat exposure to specific crash types that may be driven by heat-induced unsafe road behaviors. Here, we analyzed temperature data and police-reported crash records-including detailed crash scene information-from 177 California cities (2012-2023) using a time-stratified case-crossover design, examining variations in risk by crash type and contextual factors. We observed that extreme heat was associated with elevated risks of fatal and severe crashes, with risk increasing monotonically at higher temperatures. Associations were more pronounced in high-conflict settings such as intersections; among vulnerable road users, including bicyclists and pedestrians; and in crashes involving driver-related violations common to these contexts, such as improper turning and failure to yield to pedestrians, and instances where pedestrians or bicyclists were themselves the at-fault parties. These patterns are consistent with the possibility that extreme heat may increase crash risk by influencing road behavior, reinforcing behavioral mechanisms as a plausible pathway. They also highlight potential disparities in road safety, particularly affecting individuals who continue to walk or cycle and remain mobile during extreme heat, such as low-income individuals without access to climate-controlled vehicles. Urban planning and transportation policies should integrate climate resilience into road safety strategies, such as shaded intersections, cooling interventions for vulnerable road users, enhanced right-of-way protections during extreme heat, and institutional recognition of heat as a situational risk factor.