How does City-cluster high-speed rail facilitate regional integration? Evidence from the Shanghai-Nanjing corridor

Most existing studies of high-speed rail (HSR) have paid attention to inter-regional trunk lines. This paper presents a case study of the City-Cluster HSR (C-HSR), a particular type of HSR line serving municipalities within a city-cluster (conceptually equivalent to a mega-city region in Europe or a mega-region in the United States context). The study investigated the effects of C-HSR on regional spatial reconfiguration along the corridor between Shanghai and Nanjing (namely the Hu-Ning Corridor) located in the Yangtze River Delta, China. In particular, the study attempted to answer the following questions: What role does the C-HSR play in city-cluster growth of the corridor? Does it increase regional integration or induce polarization? Three research steps are carried out: identifying the type of travel benefiting most from the C-HSR, describing the changing paradigm of city network structure in the corridor, and revealing the mechanism of the change. Based on the perspective of a "space of flows", the passenger flows of Hu-Ning C-HSR in 2010 and 2015 were compared in terms of link intensity and aggregation intensity. The results show that the main service objects of C-HSR were secondary intermediate districts and their trips to urban districts in different municipalities. These trips provide the opportunity for potential sub-districts, especially those adjacent to regional core urban districts, to give rise to a horizontal and polycentric city network and promote regional integration. The competitive travel distance to other transits, reasonable travel time, and internal conditions are found to be the main C-HSR's effects. This ex post study uses a combination of aggregated flows and individual movements, a perspective not yet applied to the regional level, to explain the process of reconstruction of the regional spatial system. Theoretically, the findings are conducive to illustrate this emerging spatial phenomenon and presenting hard evidence of the importance of short and medium HSR service in an advanced stage of urbanization. In practical terms, they have implications for regional planning policies on C-HSR station set up, regional land use layout, and industry distribution, and provide suggestions for C-HSR operation schemes.