Boundary effects on road network vulnerability in cities

Road networks are typically connected at the national scale, thus investigating a city's road network vulnerability requires defining an urban boundary. However, the choice of boundary varies widely in research, resulting in divergent and at times contradictory findings. Moreover, analyzing larger cities' road networks with expanding boundaries helps to predict road network vulnerability patterns in smaller, similarly structured cities. Guided by these two objectives, this study investigates the gaps in road network vulnerabilities by comparing 1452 road networks from 363 cities in Mainland China across four distinct boundary types. The findings indicate a significant change in road network vulnerability across different boundaries, with the largest gap ranging between 0.5449 and 0.8743. The study further finds these vulnerability gaps correlate with the difference in the structural characteristics of those urban road networks and the urban population covered by different boundaries, with correlation coefficients ranging from 0.362 to 0.755. Notably, the cities' vulnerability rankings show relatively slight variations when adopting different boundaries, with Spearman's correlation coefficients between 0.442 and 0.879. This research offers valuable insights into the influence of urban expansion on road network vulnerability, particularly relevant in the context of rapid urbanization and the expansion of road networks.