The 15-15 rule for healthy cities: An operational subject-object integrated model to bridge greenspace accessibility and effective exposure in environmental planning.

While the health benefits of urban green spaces (UGS) are widely recognized, research often isolates the optimization of UGS spatial patterns from the analysis of population exposure effects. To bridge this gap, this study proposes a Subject-Object Integrated (SOI) model that conceptualizes the complete pathway from accessibility to effective exposure. We operationalize it as the 15-15 rule, requiring residents to reach a UGS within a 15-min walk and obtain at least 15 min of effective exposure. Using Shanghai as a case study, we quantified the UGS exposure potential at community level, evaluated supply-demand relationship, and simulated staged UGS construction under 15-15 rule. Our results indicate that approximately 80% of communities and their populations currently meet the 15-15 rule, with compliance rates decreasing from the urban center to the exurban areas. Supply-demand analysis further reveals distinct spatial patterns: the urban center shows high demand-supply consistency but relatively low average UGS provision; suburb face severe supply deficits; and exurban areas possess relatively abundant yet underutilized UGS resources. The staged construction simulation suggests that adding 499 UGS units (693.78 ha), primarily in suburb and exurban zones, would raise the compliance rate to 99.8% in a basic satisfaction stage. Subsequent priorities for quality enhancement diverge. The urban center requires expansion of existing UGS, whereas suburb and exurbs need more newly planned UGS. Collectively, the SOI model and the 15-15 rule provide a theoretically grounded and practically operational approach for environmental management in healthy-city goals.