A comparison of heat effects on road injury frequency between active travelers and motorized transportation users in six tropical and subtropical cities in Taiwan

Road traffic injuries (RTIs) pose significant public health threats, particularly for vulnerable road users such as pedestrians and cyclists. While recent studies have revealed adverse impacts of heat exposure on RTI frequency among motorized road users, a research gap persists in understanding these impacts on non-motorized road users, especially in tropical regions where their vulnerability can be heightened due to differential thermal exposure, adaptive capacity, and biological sensitivity. In this study, we compared associations between high temperatures and RTIs across four different crash-involved modes of transportation—pedestrians, cyclists, motorcyclists, and car drivers in Taiwan. Leveraging data on RTI records and temperature conditions in Taiwan's six municipalities from 2018 to 2022, we conducted a city-time-stratified case-crossover analysis. We employed distributed lag non-linear models with conditional Poisson regression models to estimate temperature-RTI associations for each mode of transportation, adjusting for various weather factors and unmeasured spatio-temporal patterns. Our findings reveal that individuals using exposed, open transportation modes (i.e., pedestrians, cyclists, and motorcyclists) exhibited higher relative risks of heat-induced RTIs than car drivers, with non-motorized mode users showing greater susceptibility compared to their motorized counterparts. These elevated risks can be attributed to the absence of built-in cooling systems in open travel modes and the increased exertional heat stress implied in active travel. Our study contributes novel insights to a global concern related to climate change, extending its impact to road safety, a health outcome rarely studied in the context of a changing climate. Our findings are thus important, especially for regions where rising temperatures regularly approach or exceed human physiological limits related to heat tolerance in the coming decades. Additionally, our findings hold significance in the existing urban health literature, particularly within the context of the emerging era of micromobility—a category of low-speed, non-enclosed, and lightweight vehicles increasingly integrated into urban activities worldwide.