From "physical repair" to "mental resonance": spatial perception and behavioral reshaping pathways in the renewal of lost under-bridge spaces.

INTRODUCTION: Urban under-bridge spaces are often regarded as typical "lost spaces" due to ambiguous boundaries, limited functions, and fragmented management. While existing regeneration practices have largely focused on functional supplementation, safety governance, and the maintenance of spatial order, the mechanisms through which physical environmental qualities are translated into public interaction behaviors via users' psychological experiences remain theoretically underspecified. METHODS: Grounded in environmental behavior theory and adopting a mixed-methods research design, this study takes the under-bridge space of the Lujiazui Waterfront Ring Bridge in Shanghai as the research object. Empirical data were collected through a pedestrian questionnaire survey, and a structural equation model (SEM) was constructed to operationalize the following theoretical pathway: Physical Environment - Mental Perception - Behavioral Reshaping. RESULTS: The analysis of 357 valid questionnaires shows that FS, AC, and RC all exert significant effects on MP (β = 0.181, p = 0.006; β = 0.221, p < 0.001; β =  0.409, p < 0.001). FS and RC also have significant direct effects on BR (β = 0.205, p = 0.001; β = 0.280, p = 0.001), whereas the direct path from AC to BR is not supported (β = 0.116, p = 0.063). The Bootstrap mediation test with 5,000 resamples indicates that MP plays a partial mediating role between FS and BR, with an indirect effect whose 95% confidence interval is [0.016, 0.138], and between RC and BR, with a 95% confidence interval of [0.056, 0.240]. In contrast, MP fully mediates the relationship between AC and BR; the 95% confidence interval for the path from AC to BR was [-0.010, 0.288], and MP plays a full mediating role between AC and BR, with a confidence interval of [0.024, 0.151].The model demonstrates good fit indices (χ2/df = 1.237, RMSEA = 0.026, GFI = 0.950, CFI = 0.988). DISCUSSION: These findings clarify the theoretically ambiguous relationship between spatial regeneration and behavioral outcomes by demonstrating that mental perception constitutes a crucial and differentially operative mediating pathway: while functional and reachability dimensions retain direct behavioral effects independent of perception, aesthetic and comfort qualities exert their influence exclusively through the psychological experience of users. Accordingly, under-bridge space regeneration should be seen as a continuous, integrative process involving physical environment upgrades, psychological experience enhancements, and social behavior generation. These findings offer empirical and theoretical insights for regenerating under-bridge spaces, reconstructing publicness, and enhancing urban space quality.