Travel efficiency in urban space: the role of built environment in shaping excess travel distance across transport modes

This study examines the relationships between three built environment factors (i.e., urban density, transportation accessibility, and neighborhood design) and excess travel distance across three transportation modes: private vehicles (CAR), public transportation (PT), and active transportation (AT) in Gunsan, a medium-sized city in South Korea. Utilizing mobile phone-based mobility data, the analysis integrates Explanatory Factor Analysis, Multilevel Regression, and Extreme Gradient Boosting with Partial Dependence Plots to identify mode-specific and context-dependent patterns in travel inefficiencies. The findings indicate that increased density at both trip origins and destinations is generally associated with reduced excess travel distance, particularly for PT and AT users; however, extremely high density levels may induce inefficiencies for CAR and PT modes. Accessibility demonstrates a counterintuitive effect, where higher accessibility correlates with greater excess travel across all modes. In terms of urban design, a U-shaped relationship emerges for PT and AT modes: excess travel initially decreases at lower levels of design quality, increases at moderate levels (e.g., mid-range compactness and POI diversity), and then declines again at higher levels. This study contributes to offering (1) a more comprehensive understanding of how urban form influences travel efficiency, and (2) implications for enhancing urban efficiency and sustainability.