## 6 Conclusions  
This study quantitatively examines the interplay between administrative fragmentation and functional integration in Jurong, a typical urban interstice in the rapidly urbanizing Yangtze River Delta. Leveraging mobile OD flow data, we systematically analyze intercity mobility, regional connectivity, and spatial interaction patterns, providing empirical insights into the dynamics of urban interstices. Jurong exemplifies the dual nature of urban interstices--absorbing metropolitan spillovers from Nanjing while remaining institutionally bound to Zhenjiang. This spatial-functional mismatch reveals how urban flows increasingly transcend administrative boundaries, challenging traditional governance frameworks. The emergence of subcenters like Baohua (residential) and Guozhuang (innovation) along infrastructure-rich corridors reflects a corridor-based, polycentric development mode. Jurong's alignment with Nanjing, despite limited support from its administrative center, further illustrates how governance fragmentation can prompt bottom-up integration strategies, though often at the cost of formal policy coherence. Theoretically, this study contributes to the evolving understanding of interstitial urbanization, particularly under state-led governance regimes. It introduces a relational perspective that emphasizes functional flows over static territorial hierarchies, situating urban interstices as dynamic mediators within fragmented spatial structures. Methodologically, the use of mobile signaling data offers a replicable approach to identifying and quantifying inter-urban linkages, providing an evidence-based lens through which to understand the real-time dynamics of urban connection. It also presents the following practical policy implications:  
* In China, the case of Jurong points to the importance of recognizing interstitial cities as strategic actors within larger urban agglomerations. Comparable cases such as Kunshan (Suzhou)-Shanghai and Pinghu (Zhejiang)-Shanghai suggest that governance and planning reforms should more explicitly account for high-mobility corridors that defy existing administrative delineations (Figure 13). This spatial selectivity underscores the necessity for targeted infrastructural investment and highlights the active agency of urban interstices in regional spatial restructuring. Tools such as joint metropolitan planning institutions, cross-jurisdictional zoning schemes, or flexible land-use frameworks can enable these areas to participate more meaningfully in regional coordination and resource allocation. * Internationally, these findings are applicable to fragmented metropolitan regions such as Cheshire East and Stockport in the UK, and Greater Tokyo in Japan, where spatial-functional integration is increasingly misaligned with administrative jurisdictions. In such systems, administrative cooperation and functional zoning can help reconcile governance mismatches with emerging spatial interactions shaped by commuting, service use, and cross-border economic flows. For example, the UK's Localism Act (2011) mandates local governments to engage in strategic planning cooperation through the \"Duty to Cooperate\" framework [65; 66], reflecting institutional efforts to mitigate fragmentation without redrawing boundaries. By employing mobile phone signaling data--similarly to the methods used in this study--planners can quantitatively capture real-time population flows across fragmented governance territories. This enables the evidence-based identification of emerging spatial-functional linkages that may not be visible through traditional administrative datasets and supports more responsive, dynamic governance models. Accordingly, the spatial strategy should match the pattern of human flows and local constraints. In areas with stable, high-volume directional commuting, infrastructure investment (e.g., transit corridors, intercity rail) can consolidate linkages.