Expressway development and income inequality: Nonlinear and spatially differentiated effects

The income distribution effects of transport infrastructure investment remain controversial. Although expressway expansion can improve connectivity and promote economic interaction, it may also reshape the spatial distribution of economic activity and widen regional income inequality. Using panel data for 30 Chinese provinces from 2010 to 2023, this study examines the effects of expressway development on the urban–rural income gap from the perspective of regional factor-carrying capacity, proxied by population density and economic development level. The results show that expressway development generally reduces income inequality, with logistics accessibility and labor mobility serving as key transmission channels. Threshold regression results reveal significant nonlinear characteristics. At relatively low levels of regional factor-carrying capacity, expressway construction tends to reinforce the concentration of production factors in core regions and widen income inequality. As regional factor-carrying capacity improves, diffusion effects gradually strengthen and contribute to more balanced income distribution. However, the marginal inequality-reducing effects of further expressway expansion weaken in regions with relatively mature transport networks and economic systems. GTWR results reveal substantial spatiotemporal heterogeneity. These findings help reconcile inconsistent conclusions in prior literature, highlighting that the distributional effects of expressway investment are highly conditional on regional development and spatial economic structures.