The double-edged sword of the 15-minute city: Mediation and moderation effects of built environment on social segregation

Abstract

The 15-min city is widely promoted as a model for sustainable and livable urban development, but its implications for social segregation remain unclear. Using anonymized mobile phone trajectory data of 8.6 million residents in Shenzhen, China, this study examines whether localized daily activity patterns foster social integration or reinforce socioeconomic homogeneity in everyday encounters. We measure localized living as the share of non-work activities conducted within a 15-min walking radius of home, and characterize the neighborhood built environment by density, diversity, and connectivity within the same catchment. Activity-based social segregation here measures the degree to which individuals are exposed to others with similar socioeconomic status during daily activities. The results reveal that a higher 15-min activity share is associated with greater social homogeneity. Density reinforces this pattern indirectly by encouraging more localized activities, whereas amenity diversity plays a distinct moderating role. When neighborhood diversity exceeds a threshold (Shannon entropy > 0.84), the association reverses, indicating that highly mixed-use environments can mitigate the “social bubble” effect of localized living. These findings show that proximity alone does not ensure social integration. The social consequences of the 15-min city depend critically on neighborhood opportunity structures, especially amenity diversity.