| Entering the new era,urban agglomerations have become the basic geographical unit for the country to participate in global competition and international division of labor,and it is also the main form of China’s new urbanization and an important carrier for promoting regional integrated development.Transportation is an important part of the structure of urban agglomerations,and it is also an important support for the integrated development of urban agglomerations.It is of great significance to guide the spatial development of urban agglomerations and optimize the spatial structure of urban agglomerations.In March 2021,the "14th Five-Year Plan for National Economic and Social Development of the People’s Republic of China and the Outline of Vision 2035" clearly stated that it is necessary to optimize the internal spatial structure of urban agglomerations,and form a multi-center,multi-center Hierarchical,multi-node network urban agglomeration.With the rapid development of information technology,the connection between cities has been continuously strengthened,and the networking of urban agglomerations has become an inevitable trend to reshape the regional spatial pattern.The emergence and development of the concept of "flow space" has made the geography community pay more attention to the research on the network structure of "flow" in urban agglomerations.It is hoped that the spatial characteristics of social and economic development of urban agglomerations and the connection between cities can be more deeply understood,so as to improve the scientificity and rationality of spatial planning of urban agglomerations and promote the integrated development of urban agglomerations.As an important form of "flow space" network,traffic flow is an important carrier of various production factors such as population,goods,capital and technology in the region.It can directly reflect the connection density,intensity and hierarchical structure between cities,so it has become an important means to study the network structure of urban agglomerations.The western region occupies an important strategic position in the survival and development of the Chinese nation.In May 2020,the Central Committee of the Communist Party of China and the State Council issued the "Guiding Opinions on Promoting the Western Development and Forming a New Pattern in the New Era",marking the entry of the Western Development into the 3.0 era."Opinions" request,"Promote high-quality development in the western region","strengthen the construction of transportation corridors that traverse east-west and north-south","promote the coordinated development of Chengdu-Chongqing urban agglomeration and Guanzhong Plain urban agglomeration","promote Beibu Gulf,Lanzhou-Xining,Hohhot-Baotou-Ordos-Yulin,Interactive development of the Yellow River in Ningxia,Central Guizhou,Central Yunnan,and the northern slope of the Tianshan Mountains”.The Fourteenth Five-Year Plan for the National Economic and Social Development of the People’s Republic of China and the Outline of the Vision for 2035 also call for"promoting the integrated development of urban agglomerations".Under the action of different modes of transportation,the spatial structure of urban agglomerations has evolved into different forms.Therefore,the research on the spatial structure of urban agglomerations based on the traffic flow network is conducive to grasping the differentiated characteristics of the spatial structure of nine urban agglomerations in the western region,and clarifying the advantages and disadvantages of spatial development of different urban agglomerations.It is of great significance to optimize and expand the development axis of the western region.Therefore,based on the theoretical framework of urban network,this paper takes 9urban agglomerations in the western region as the research objects,and 72prefecture-level and above administrative units(prefecture-level cities,autonomous prefectures,municipalities directly under the Central Government,regions)as the basic research units.Based on highway traffic flow,ordinary-speed railway traffic flow,high-speed railway traffic flow and comprehensive traffic flow,the O-D matrix of traffic connections within urban agglomerations is constructed,and technical means such as social network analysis,complex network analysis and GIS visualization analysis are used.The network structure characteristics and spatial heterogeneity of the nine urban agglomerations are described from three dimensions: attribute characteristics of urban nodes within urban agglomerations,inter-city connection characteristics within urban agglomerations,and network topology characteristics of urban agglomerations.Finally,based on the weighted comprehensive traffic flow,the cluster analysis of the network topology of the urban agglomeration is carried out,and the following conclusions are drawn:(1)From the characteristics of urban nodes presented by a single traffic flow.The urban node centrality of the nine urban agglomerations in the highway flow,the ordinary-speed railway flow,and the high-speed railway flow all show a strong polarization phenomenon.The centripetal polarization of highway flow is concentrated in the central cities of each urban agglomeration,especially in the five urban agglomerations of Chengdu-Chongqing,Central Guizhou,Central Yunnan,the Yellow River in Ningxia,and Lanzhou-Xining.In addition to the polarization of central cities,the centrality of secondary central cities has begun to emerge.In particular,Yulin and Baotou have become the cities with the highest centrality in the Beibu Gulf,Hohhot-Baotou-Ordos-Yulin.The polarization of urban node centrality reflected by the rapid railway flow is the most prominent,and the centrality of some central cities(such as Hohhot and Urumqi)has accounted for 50% of the total urban agglomeration.The three types of traffic flow present different spatial forms of node centrality.The high-value areas of urban centrality for the ordinary-speed railway flow are obviously more than those for the highway flow,and there has been a “clump-like” continuous distribution between adjacent cities.However,the high-value areas of high-speed railways are more widely distributed than ordinary-speed railways,and are scattered in9 urban agglomerations in the form of "points" or "clumps".The three traffic flows present asymmetrical external traffic directions of city nodes.This is manifested in the fact that there are more cities with a point-in degree than a point-out degree,and66.67% of the cities have a higher point-in degree of highway flow than a point-out degree,and 57.50% and 54.90% of the cities that have opened ordinary-speed railways and rapid railways.(2)From the characteristics of inter-city connection intensity reflected by a single traffic flow.The skeleton network composed of high-strength connections in the highway flow network has appeared in five urban agglomerations including Chengdu-Chongqing,Guanzhong Plain,Beibu Gulf,Central Guizhou,and Lanzhou-Xining.Among them,an "umbrella" structure centered on Chengdu and Guiyang is formed in the urban agglomerations of Chengdu and Chongqing and Central Guizhou.The high-strength inter-city connections in the Guanzhong Plain,Beibu Gulf,Lanzhou-Xining have not yet formed a network,and only a few skeleton axes have emerged.However,there are 4 urban agglomerations such as Central Yunnan,Hohhot-Baotou-Ordos-Yulin,the Yellow River in Ningxia,and the northern slope of Tianshan Mountains,which have not yet emerged as skeletons representing high-strength connections.The network skeleton development degree of ordinary-speed railway flow is lower than that of highway flow network,and it only appears in three urban agglomerations such as Hohhot-Baotou-Ordos-Yulin,Guanzhong Plain,and Lanzhou-Xining,and also forms an “umbrella” structure radiating outward from the central city.The network skeleton of the rapid railway flow is relatively well developed,and it has appeared in the six urban agglomerations of Chengdu-Chongqing,Guanzhong Plain,Beibu Gulf,Hohhot-Baotou-Ordos-Yulin,and Central Yunnan.Moreover,the radiation range is larger than that of road flow and ordinary-speed railway flow,and most of them have a "radial" shape that spreads from the central city to the surrounding cities.(3)From the perspective of the topological structure characteristics of the internal network of the urban agglomeration represented by a single traffic flow.The average density of the highway flow network in the nine urban agglomerations is higher than that of the ordinary-speed railway flow and the high-speed railway flow,which also shows from the side that the highway network in the western region is more developed than the other two types of transportation.The degree of connection between cities is related to the size of the urban agglomeration.The three types of traffic flow connections in the smaller urban agglomerations,such as central Guizhou,central Yunnan,Ningxia,Yanhuang,Hubao and Eyu,are more connected than those of larger cities such as Chengdu-Chongqing,Guanzhong Plain,Lan-xi,Beibu Gulf,and the northern slope of Tianshan Mountains.Group tight.The three types of traffic flow connections in the smaller urban agglomerations,such as Central Guizhou,Central Yunnan,the Yellow River in Ningxia,Hohhot-Baotou-Ordos-Yulin,are more connected than those of larger cities such as Chengdu-Chongqing,Guanzhong Plain,Lanzhou-Xining,Beibu Gulf,and the northern slope of Tianshan Mountains.The three types of traffic flow networks have different degrees of development,and the highway flow network is relatively developed.Among them,four urban agglomerations such as Chengdu-Chongqing,Central Guizhou,Hohhot-Baotou-Ordos-Yulin,and the Yellow River in Ningxia have intertwined urban connections,showing a circuit-like network.The network development degree of ordinary-speed railway flow and rapid railway flow is still relatively low.Among the ordinary-speed railway flow,only the ring formation rate of the urban agglomeration of Hubao and Eyu reaches 1,which is a loop-like X network structure.However,the high-speed railway flow looping ratios of the nine urban agglomerations are all less than 1,all of which are tree-like network structures.The intensity,symmetry and hierarchy characteristics of the three traffic flow network topologies are basically similar in the nine urban agglomerations,and five urban agglomerations including Hohhot-Baotou-Ordos-Yulin,Central Guizhou,Central Yunnan,the Yellow River in Ningxia,and Chengdu-Chongqing performed better.(4)From the perspective of the spatial structure characteristics of the urban agglomeration network represented by the comprehensive traffic flow.The difference of node centrality between cities is smaller than that of single traffic flow,but the heterogeneity of spatial distribution is still obvious.Among them,five urban agglomerations,including Guanzhong Plain,Beibu Gulf,Lanzhou-Xining,Hohhot-Baotou-Ordos-Yulin,and the northern slope of Tianshan Mountains,respectively formed a "clump-like" agglomeration centered on Xi-an,Nanning,Lanzhou,Hohhot,and Urumqi.The high-value areas of centrality in the urban agglomeration along the Yellow River in Chengdu-Chongqing,Central Guizhou,Central Yunnan,and the Yellow River in Ningxia are clustered in a "point-like" manner.The urban agglomeration network skeleton reflected by comprehensive traffic flow appears in more urban agglomerations than a single transportation mode,only the two urban agglomerations along the Yellow River in Ningxia and the northern slope of Tianshan Mountain have not yet formed a network skeleton.However,the Chengdu-Chongqing urban agglomeration and the Guanzhong Plain urban agglomeration have initially formed a radial skeleton network space form centered on Chengdu and Xi-an.The topological structure characteristics of the urban agglomeration network presented by the integrated traffic flow have better spatial coupling with the single traffic flow.The three urban agglomerations of Chengdu-Chongqing,Central Yunnan,and Guanzhong Plain have a relatively high degree of internal network development,and a prototype of a complex network has already been established.The northern slope of the Tianshan Mountains,Lanzhou-Xining,Beibu Gulf,Hohhot-Baotou-Ordos-Yulin,the Yellow River in Ningxia,and Central Guizhou are still relatively loosely connected,and most of them show strong centripetal characteristics of central cities.(5)Based on the compactness,intensity,symmetry,hierarchy,and degree of network development of the integrated traffic flow network topology,the nine urban agglomerations in the western region are divided into four types of network space.That is to say,"a dual-core urban agglomeration with strong hierarchy and well-developed network",including the Chengdu-Chongqing urban agglomeration.The "Monocentric urban agglomerations with strong hierarchy and strong connections",including the Guanzhong Plain urban agglomeration and the Central Yunnan urban agglomeration.The "centripetal urban agglomeration with weak hierarchies and weak connections" includes urban agglomerations such as the northern slope of the Tianshan Mountains,Lanzhou-Xining,and Beibu Gulf. |
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