Heat stress and active travel, an eight-year national analysis in The Netherlands.

Rising temperatures and more frequent heat events may affect the uptake of active travel modes such as walking and cycling, with implications for public health and sustainable mobility. We examined how perceived temperature, measured via the Universal Thermal Climate Index (UTCI), is related to walking and cycling behavior in The Netherlands using a national travel questionnaire spanning eight summers (2016-2023; n = 178,247). We employed linear splines to separately assess association between UTCI and active travel in general, and the associations above predefined heat stress thresholds that were chosen based on UTCI categories (26°C and 32°C). A Heckman selection model was used to account for the effect of UTCI both on the decision to travel and on the amount of active travel, distinguishing between three trip purposes and between walking and cycling. We found a non-linear relation: effect of temperature on active travel uptake is positive, but beyond the threshold of 32°C it becomes negative. A similar pattern emerges for the degree in which people travel in the first place. These associations were found for all-purpose and recreation/sports-related trips, but not for work-related trips, suggesting that work-related travel is less flexible to heat. We found some evidence for an alleviating effect of tree canopy cover on the negative impact of heat, but not consistently across modes and trip purposes. Our findings highlight the need for policies that support heat-resilient active travel and that allow work commuters to have more heat adaptation options.