What makes older adults transfer? built environment drivers of metro–bus interchange: a case of Suzhou, Jiangsu, China

• A natural policy experiment in Suzhou revealed built environment effects on older adults’ transfer behavior. • Fare incentives had no direct effect on senior cardholders but altered multimodal travel dynamics. • Mid-level barrier-free facilities and short walking distances significantly influenced interchange rates. • Spatial heterogeneity of behavior suggests localized infrastructure priorities for demand management. • Findings advocate for non-price strategies to enhance inclusive and accessible transport systems. Population ageing is reshaping public transport demand, yet the “last transfer” between rail and bus remains a persistent weak point for older adults, where small frictions (such as distance, navigation, step-free access) can negate the intended benefits of multimodal systems. This study examines how built environment factors influence metro-to-bus interchange behavior among older adults in Suzhou, China, utilizing the implementation of a citywide fare discount policy as a natural policy experiment. Although older adults were not direct beneficiaries of the discount due to existing free ride privileges, the policy context altered multimodal travel conditions citywide. Leveraging smart card transaction data and spatial indicators, we employed a nonlinear machine learning model and SHAP-based interpretation to identify key determinants of transfer behavior. Findings show that interchange rates among older adults increased by 58.41% on weekdays and 34.78% on holidays, with significant spatial variation across metro stations. Critical factors included mid-level barrier-free infrastructure, walking distances of under 200 m, and accessible bus service to recreational destinations. The results highlight the importance of integrating environmental and service-based strategies into demand-side mobility management—beyond price incentives—to improve accessibility for older populations and inform inclusive transit design.