Reinterpreting 15 Min Urban Park Green Space Accessibility in High-Density Built Environments: Park Hierarchy and Threshold Transition in Tianjin, China

Urban park green space (UPGS) accessibility within 15 min is widely used to characterize neighborhood-scale park provision, yet aggregated accessible park supply (APS) does not reveal its hierarchical composition or the additional supply gained as walking thresholds expand. This limitation motivates a park-hierarchy and threshold-transition framework for diagnosing the internal structure of 15 min accessible park supply. In Tianjin’s central urban area, the analysis linked 63 subdistricts to 77 polygon-based UPGS destination records representing 72 park sites through 4851 origin–destination walking-time pairs and compared 10, 15, and 20 min thresholds. Parks were grouped into three analytical service hierarchies: small neighborhood-scale, medium community-scale, and large urban-scale parks. Hierarchical accessible park supply (HAPS) decomposes APS by hierarchy; APS, the Large-Park Dependence Index (LPD), and the Neighborhood Service Support Index (NSS) define four configurations, while the Threshold Gain Index (TGI) interprets threshold transition. Although 41 subdistricts had positive 15 min APS, only 17 were neighborhood-supported; 14 were threshold-compensated, 10 were large-park-dependent, and 22 had accessibility deficits. Positive APS can therefore represent different supply structures without indicating actual service sufficiency. The framework complements demand-sensitive accessibility methods. Future research should integrate population demand, park quality, actual use, population-weighted origins, park entrances, and cross-city validation.