Quantifying walkability's non-linear and synergistic effect on metro station area vitality: An empirical study in Shanghai

High-vitality metro station areas significantly enhance public transit efficiency, reduce energy consumption, and improve public life. With walkability closely linked to station area vitality, pedestrian-friendly environments are fundamental to developing vibrant nodes. While research on built environments and urban vitality is extensive, studies specifically examining walkability-vitality relationships in metro station contexts remain limited. Existing research mainly focuses on functional and accessibility aspects, overlooking the effect of public spaces and building forms that shape walking experience while neglecting potential nonlinear effects. This study takes 274 metro stations in Shanghai as examples and applies the GBDT machine learning model to explore the non-linear and synergistic effects of walkability on station area vitality from four dimensions: land use, street network, public space configuration, and building form. Results demonstrate that diverse and high-density POI, adequate squares and pedestrian malls, and short street lengths constitute the primary factors enhancing metro station area vitality. The influence of floor area ratio, motorway width, and quantity of squares on vitality exhibits pronounced non-linear characteristics, with optimal threshold ranges significantly augmenting station area vitality. Additionally, synergistic interactions are observed among several walkability indicators. The research integrates these findings into specific station area optimization strategies, further discussing the practical application value of the research findings, such as developing targeted urban renewal strategies.