Multi-layered segregation in urban transportation systems

Urban segregation is increasingly understood as a dynamic process reproduced through daily mobility [1–4], yet existing approaches often treat cross-group encounters as occurring within a single or aggregated mobility layer. In reality, sidewalks, roads, buses, and rail often traverse the same physical corridors while carrying distinct residential socioeconomic status (SES) groups, creating parallel layers of mixing and isolation [5–7]. Here we develop a framework to quantify this layered segregation and apply it to a mobile phone dataset covering 11 million individuals in Beijing, with core analyses based on 6.05 million individuals for whom stable residential and workplace anchors could be inferred. We introduce two indices: the mode-specific mixing index ( MMI ) to measure within-mode diversity, and the multimodal uniformity index ( MUI ) to summarize how evenly mixing is distributed across transport layers. We find that segregation varies sharply by transport mode: rail produces the highest observed mixing by linking disparate neighborhoods, while active travel remains locally sorted. An OD-preserving null model further shows that rail has the smallest mixing deficit relative to the level expected under Beijing’s existing origin–destination, timing, routing, and mode structure, whereas other layers show stronger residential SES sorting. This multi-layered segregation also fluctuates with daily rhythms, becoming more pronounced in peripheral zones and during commuting hours where directionally organized mobility flows amplify social divergence. Exploratory mode-choice simulations suggest that changes in generalized travel costs can redistribute residential SES groups unevenly across mobility layers. This research reveals the social dimensions inherent in transport systems, providing metrics to incorporate modal stratification into urban planning and policy evaluation.