Nonlinear mobility responses to heat in the United States: benchmark thresholds and neighborhood inequality

Extreme heat is commonly defined using climatological thresholds, but such definitions do not directly capture when daily activity begins to weaken under thermal stress. This study estimates a mobility-based behavioral benchmark for heat and examines whether post-benchmark mobility decline varies across neighborhood socioeconomic contexts in the contiguous United States. Using anonymized census tract-day mobility data from 2018 to 2024, linked to daily weather conditions, we analyze warm-season mobility responses to temperature with high-dimensional fixed effects, spline Poisson models, and piecewise Poisson threshold models. The pooled national mobility-temperature relationship is nonlinear. Predicted mobility rises through a moderate warm range, peaks near 26.7°C, and then declines at higher temperatures. In the pooled piecewise model, the benchmark turning point is estimated at approximately 23.0°C, with an uncertainty band of 22.0–26.0°C. Income-stratified models show that heterogeneity is expressed more clearly in post-benchmark slope magnitude than in benchmark location, with the steepest post-benchmark decline appearing in the lowest-income quartile, while higher-income quartiles show more moderate declines. Tract-level maps of post-benchmark heat exposure and modeled mobility burden further show that larger modeled burdens appear across parts of the South and Southwest, although substantial within-region variation remains. These results suggest that behavioral disruption under heat reflects thermal exposure, local context, and unequal neighborhood-level capacity to sustain routine activity once temperatures move beyond a broadly shared benchmark. This approach complements climatological heat definitions with an empirically estimated mobility-sensitive threshold.