Optimizing subway-based TOD: identifying threshold effects among station-level parameters

Optimizing subway-based transit-oriented development (TOD) requires strategic alignment of transit infrastructure, land use balance, and multimodal accessibility. Shapley additive explanations (SHAP) and partial dependence plot (PDP) analyses are used to identify critical saturation thresholds for station-level TOD parameters. The results reveal significant variations in TOD effectiveness on the basis of subway network density in the urban context of five major cities in South Korea. In high-density cities, subway efficiency is primarily related to multimodal integration, particularly efficient bus-subway transfers and pedestrian connectivity. In medium-density cities, a poor land use mix negatively impacts ridership, highlighting the need for balanced station-area development. In hybrid-density cities, subway efficiency depends on optimal station exit configurations and controlled parking availability rather than multimodal connectivity alone. SHAP analysis is used to identify the key determinants of subway ridership, whereas PDP analysis is applied to determine saturation effects, revealing optimal thresholds related to station exits, parking availability, and land use mix. These thresholds can help planners avoid inefficient overinvestment. The findings emphasize the necessity of city-specific TOD strategies, indicating that uniform national policies yield suboptimal outcomes. This study provides data-driven insights that enable planners to align subway investments with regional mobility demands, ensuring sustainable urban transit development.