The advancements in global urbanisation have significantly elevated the configuration of transportation networks for the sustainable development of urban agglomeration spatial structures (UASS). Using a comprehensive panel dataset covering 140 counties and districts in the Chengdu-Chongqing Economic Circle from 2014 to 2023, this study empirically examines the impact of transportation networks on the sustainability of UASS. It elucidates the underlying mechanisms using spatial Durbin, spatial spillover and multiple mediation effect models. The findings reveal that transportation networks considerably enhance the sustainability of UASS and generate substantial positive spatial spillover effects. The decay boundary of the spatial spillover effects exhibits a transition from positive to negative, eventually approaching ineffectiveness. The heterogeneity analysis indicates significant variations in terms of transportation network expansion and regional functional division on the sustainability of UASS across regions with various levels of economic development. This outcome suggests that regional development policies should carefully consider regional economic development disparities. The mechanism analysis further reveals that transportation networks enable the sustainability of UASS by accelerating regional hierarchical systems and regional connectivity.
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