Spatial Spillover Effect of High-Speed Rail Opening on Urban Sprawl in China: Evidence from a Quasi-Natural Experiment

High-speed rail (HSR), as one of the most important modern transportation tools, has accelerated population mobility and land development, thereby influencing urban spatial structure. This paper utilizes panel data from 289 Chinese cities from 2003 to 2020 and employs the spatial difference-in-differences model to examine the direct and indirect effects of HSR on urban sprawl. Our research finds that HSR significantly promotes local urban sprawl, leading to an average increase of approximately 0.027 in the urban sprawl index. Simultaneously, it has a significant inhibitory effect on urban sprawl in neighboring areas, where the urban sprawl index decreased by approximately 0.541. Furthermore, the spatial decay boundary of this inhibitory effect is 500 km. Heterogeneity analysis reveals that the promotional effect of HSR on local urban sprawl is stronger in cities with larger populations. Furthermore, it indicates that the indirect effect on neighboring urban sprawl is more pronounced in cities with higher administrative levels, located in the central and western regions, and those with smaller administrative areas. Analysis of the underlying mechanisms reveals that HSR promotes local low-density expansion by increasing housing prices and expanding urban construction land area. Furthermore, it indicates that the spatial spillover effects observed in neighboring cities are associated with an increase in industrial enterprises and permanent residents, which contributes to spatial agglomeration in those areas. This paper offers new perspectives for local governments in the context of the HSR era regarding coordinating and promoting coordinated regional spatial development, effectively managing urban sprawl, and formulating differentiated land-use and industrial policies.