Navigating the impact of metro network on bus ridership: Insights into the interdependency between metro and bus systems

The expansion of metro networks is widely acknowledged for its substitution effect on bus ridership; however, the mechanisms through which metro and bus systems interact remain insufficiently explored. This study examines the role of metro-bus interdependency in influencing urban bus ridership across 284 Chinese cities from 2008 to 2021. Utilizing a staggered difference-in-differences (DID) approach, we quantify three dimensions of interdependency—spatial overlap, service equilibrium, and service-match capability—to evaluate their respective impacts on bus ridership. The findings indicate that the introduction of the metro significantly reduces bus ridership, particularly within 1000 m of metro stations, with this substitution effect intensifying over time and reaching its peak approximately five years after metro operations commence. Nonetheless, strategic integration between metro and bus systems can mitigate this effect. Intermodal complementarity is enhanced when metro stations are strategically deployed across both well-served and underserved bus areas, and when the service levels of metro and surrounding bus networks are well matched. Furthermore, the interdependency effect exhibits distance decay, with the strongest substitution occurring within 100 m and the most significant complementary effects manifesting beyond 800 m. These findings underscore the importance of data-informed transit planning to enhance multimodal integration and minimize modal cannibalization in urban public transport networks.