Built environment impacts on bus-metro intermodal trips considering passenger trip generator features: a new method to delineate bus catchment areas for metro stations

Understanding intermodal trip generation is crucial for enhancing integrated public transit systems in megacities. This study investigates how the built environment affects bus-metro intermodal trips at the metro-station level. Existing research often overlooks defining bus catchment areas (BCAs) for metro stations and fails to account for uneven demand distribution or address catchment overlaps. Using large-scale smart-card data, this study proposes a BCA delineation method that reflects passengers’ actual travel choices. Built‑environment factors across multiple spatial ranges—including pedestrian catchment areas and BCAs—are then extracted to analyze their linear and nonlinear effects on intermodal ridership using generalized additive models. Two indicators, inner and outer radius, are further introduced to explore bus-metro intermodal patterns. A case study using two cross-sectional datasets from Beijing, China reveals consistent findings: (1) the proposed BCA delineation method outperformed benchmark methods in modeling intermodal ridership; (2) urban BCAs were unexpectedly larger than suburban ones, a finding not reported in previous studies; (3) metro stations served medium- and long-travel bus riders alongside short-travel riders; (4) population and land-use density in BCAs contributed most to the model, followed by metro accessibility, while transfer convenience contributed least. The proposed analytical framework provides a systematic tool for analyzing multimodal connectivity and informing bus–metro integration policies.