Does Public Transportation Infrastructure Always Improve Air Quality? Supply-Side Evidence on Spatiotemporal Heterogeneity, Nonlinearities, and Mechanisms from Chinese Cities

Does public transportation infrastructure expansion necessarily improve urban air quality? Using panel data from 168 Chinese cities, this study examines the impact of public transportation infrastructure development on air quality by applying GTWR (Geographically and Temporally Weighted Regression) models to capture spatial–temporal heterogeneity. Partial Dependence Plots (PDPs) are further employed to identify nonlinear relationships, alongside mechanism analysis. The results indicate that the effects of public transportation infrastructure on air quality are significant but highly heterogeneous across cities and over time. Transport development is associated with air quality through channels related to industrial transformation and agglomeration dynamics, with the latter showing a stronger relationship. Moreover, several key variables exhibit nonlinear relationships with identifiable threshold effects. These findings suggest that the environmental benefits of public transportation infrastructure are context-dependent rather than universal. This study provides a more comprehensive understanding of transport–environment linkages and offers policy insights for optimizing urban transport systems and promoting sustainable development.