sustainability Article Space-Time Evolution Analysis of the Nanjing Metro Network Based on a Complex Network Wei Yu 1 , Jun Chen 2,* and Xingchen Yan 1 1 College of Automobile and Traffic Engineering, Nanjing Forestry University, Longpan Road 159#, Nanjing 210037, China; [email protected] (W.Y.); [email protected] (X.Y.) 2 School of Transportation, Southeast University, Si Pai Lou 2#, Nanjing 210096, China * Correspondence: [email protected] Received: 11 December 2018; Accepted: 15 January 2019; Published: 19 January 2019 Abstract: Many cities in China have opened a subway, which has become an important part of urban public transport. How the metro line forms the metro network, and then changes the urban traffic pattern, is a problem worthy of attention. From 2005 to 2018, 10 metro lines were opened in Nanjing, which provides important reference data for the study of the spatial and temporal evolution of the Metro network. In this study, using the complex network method, according to the opening sequence of 10 metro lines in Nanjing, space L and space P models are established, respectively. In view of the evolution of metro network parameters, four parameters—network density, network centrality, network clustering coefficient, and network average distance—are proposed for evaluation. In view of the spatial structure change of the metro network, this study combines the concept of node degree in a complex network, analyzes the starting point, terminal point, and intersection point of metro line, and puts forward the concepts of star structure and ring structure. The analysis of the space-time evolution of Nanjing metro network shows that with the gradual opening of metro lines, the metro network presents a more complex structure; the line connection tends to important nodes, and gradually outlines the city’s commercial space pattern. Keywords: complex network; metro; space-time; evolution 1. Introduction The rapid development of China’s economy has promoted the process of urban construction. Many cities have opened subways as an important means of improving the level of urban traffic and changing the urban spatial pattern. Urban metro networks can be abstracted as metro stations and lines, and on this basis, a complex network can be formed. How the urban metro network evolves into a part of the urban public transport network, and then affects the urban spatial pattern, is a problem worthy of study. Table1 shows a list of existing relevant research on metro network evolution. The research on the evolution of transportation network includes the establishment of models, the analysis of development stages, and the evolution process. Malinetskii et al. (2009) [1] proposed a dynamic model of traffic network evolution, whose dynamic behavior is the result of system self-organization. Ma et al. [2] (2015) tried to extend the theory of in-depth learning to the analysis of large-scale traffic network, and used the data from a taxi global positioning system to model and predict the evolution of traffic congestion. Chen et al. (2017) [3] discussed the long-term evolution characteristics of land transportation in the Beijing-Tianjin-Hebei area, and explained the five development stages of land transportation network by using a time scale of 100 years. Liu et al. (2018) [4] studied the influence of travel time selection on the daily dynamic evolution of traffic flow in traffic network, and studied the evolution process of network traffic flow by numerical experiments. Sustainability 2019, 11, 523; doi:10.3390/su11020523 www.mdpi.com/journal/sustainability Sustainability 2019, 11, 523 2 of 17 Table 1. A list of existing relevant research on metro network evolution. Number Author Main Work 1 Malinetskii, G.G. Dynamic model of traffic network evolution 2 Ma, X. Modeling and predicting the evolution of traffic congestion 3 Chen, Y. Discussing the long-term evolution characteristics of land transportation 4 Liu, S. Studying the evolution process of network traffic flow by numerical experiments 5 Wandelt, S. Analyzing the evolution of the national network of international air transport 6 Li, Z. Studying the time evolution of European air transport system 7 Wu, X. Analyzing the characteristics and dynamic changes of the spatial distribution of air transport utilization 8 Kim, J. Analyzing the spatial evolution of Seoul Railway Station in Korea 9 Jiang, Y. Proposing a bus maintenance strategy based on complex evolutionary method 10 Mohammed, A. Describing a simulation model of bus network evolution based on GIS 11 Chen, X. Proposing the average annual bus trip per capita and the average daily trip per bus of bus system 12 Huang, A. Studying the intrinsic attributes and evolution mechanism of urban public transport network 13 Wu, J. Proposing a dynamic evolution model of daily movement in urban railway network 14 Leng, B. Analyzing the evolution of Beijing metro network in the process of development 15 Kim, H. Discussing the four evolution stages of the Seoul metro system A typical network of urban external traffic includes the aviation network and the railway network. Wandelt et al. (2015) [5] analyzed the evolution of the national network of international air transport from 2002 to 2013 from the perspective of network physical topology and the traffic information functional network. Li et al. (2015) [6] studied the time evolution of European air transport system, analyzed the air navigation routing network and airport network, and found that the hub node existing in two network layers is the bottleneck of network development. Wu et al. (2018) [7] analyzed the characteristics and dynamic changes of the spatial distribution of air transport utilization in China’s provinces and regions, and established a multiple linear regression model to forecast future airport passengers and cargo volume and the throughput growth rate of each province. Kim (2009) [8] analyzed the spatial evolution of Seoul Railway Station in Korea, and revealed the mutual adaptation between the architectural elements of Seoul Railway Station and the urban environment. The most common traffic network within a city is the bus transport system network. Jiang et al. (2012) [9] proposed a bus maintenance strategy based on the complex evolutionary method, and established a mathematical model with the objective of minimizing the waiting time of passengers at the current station and the next station. Mohammed et al. (2013) [10] describe a simulation model of bus network evolution based on Geographic Information System (GIS), which successfully identifies bus lines prone to frequency changes. Chen et al. (2014) [11] proposed the average annual bus trip per capita and the average daily trip per bus, describing two aspects of bus system performance: travel intention and operation efficiency. Based on these two indicators, a variable coefficient model was used to analyze the impact of each variable on the travel intention and operational efficiency of different types of cities. Huang et al. (2016) [12] studied the intrinsic attributes and evolution mechanism of urban public transport network, and proposed a two-level evolution model to simulate the development of public transport lines from the perspective of complex networks. Some studies involve the travel characteristics, growth patterns, and evolution stages of metro network evolution. Wu et al. (2013) [13] proposed a dynamic evolution model of daily movement, which can better reflect the characteristics of pedestrian travel in an urban railway network. Leng et al. (2014) [14] analyzed the evolution of the Beijing Metro Network in the process of development, proposed a new growth model consisting of expansion model and strengthening model, and defined and evaluated the network using a line-weighted network. Kim et al. (2015) [15] discussed the four evolution stages of the Seoul metro system from the vertical changes of network accessibility and reliability. With the rapid expansion of the network, accessibility and reliability increase with the passage of time, but with different speeds, spatial patterns are also different. Many networks can be simplified into nodes and relationships among them, which constitute the basic elements of complex networks. The small-world effect of complex networks means that most of the nodes are very close to each other [16]. Public transport network is also a special complex network. Transportation stations or hubs constitute nodes, and transportation routing constitutes the relationship between nodes [17–20]. As a part of an urban public transport network, the study of Sustainability 2019, 11, 523 3 of 17 the metro network includes the modeling of urban metro network, and on this basis, the structural parameters and the relationship between the metro network and traffic are analyzed. There are also some studies on how to keep the subway network stable when attacked [21–24]. Metro construction and investment have a long-term nature, and sustainable development must be considered. Once the metro system is built, it is very difficult to renew and reform it. With the continuous development of society, the travel needs of residents may also change. The planning of a metro network needs to be carried out in stages and adjusted continuously. The existing research on metro networks focuses on the evaluation of network performance and robustness when attacked. When studying the evolution of metro networks, the related articles often discuss an abstract evolution stage, rather than evaluating it with specific parameters. The Nanjing metro network mentioned in the paper is still in the stage of rapid development, and the metro data provided by Nanjing Metro Co., Ltd. are relatively clear and can be related to the actual development of urban land and analysis. On this basis, the evolution analysis of Nanjing metro network has practical reference value. Nanjing, as a developed city in China, takes subway planning as an important part of urban traffic network. From 2005 to 2018, a total of 10 metro lines were opened in Nanjing, and more and more metro lines are under construction.