Complex Network Property And Vulnerability Of Urban Rail Transit Networks

Rail transit has many advantages,such as high speed,large capacity and strong security,so that it becomes one of the most effective methods to alleviate the urban traffic jam.In recent years,with increasing passenger flow and construction scale,metro systems in metropolises such as New York,Tokoyo,Shanghai,etc.,have entered a new era of networking operation.However,frequent occurrences of random failures and malicious attacks pose serious threats to metro security and reliability.Thus,it is necessary to quantitatively evaluate the vulnerability of the metro network to different failures or attacks from a networking perspective.The present study focuses on the urban rail transit systems and investigates the safety and vulnerability of rail transit networks with the use of the theories and methods of network science,traffic engineering and statistics,the details are as followsFirstly,based on the theory of complex network,the characteristics of unweighted(topological)and weighted complex network of rail transit were analyzed to understand the complexity and evolution law of rail transit network.The weighted complex model was established by considering the passenger flow and spatial constraint to analyze the distribution laws of node degree,edge weight and node strength of Shanghai rail transit(SRT)network.Meanwhile,the connectivity and assortativity of SRT network were also discussed.Then,the similarity and the difference between statistical conclusions on unweighted and weighted networks were compared.The results showed that the weighted network integrating passenger flow and geographic distance could correctly reflect the characteristics of rail transit network.However,there were errors between statistical conclusions obtained from the unweighted network and the real network owing to the neglect of dynamic passenger flow and geographic distance.Secondly,the vulnerability of weighted and topological(unweighted)complex network was invesgated to explore the impact of different attack strategies and scenarios on the unweighted and weighed network of rail transit.Network vulnerability of rail transit was defined and invesgated in two aspects,i.e.,the relative size of maximal connected subgraph and the golbal network efficiency.Then,the evaluation model of rail transit network vulnerability was established.Taking the SRT network as an example,the vulnerability of unweighed and weighted SRT network subjected to various attack strategies was evaluated,meanwhile,the impact of single station failure,multiple stations failure and continuous failure on the SRT network were evaluated.The results showed that the topological and weighted highest betweeness attack strategies were the most effective modes to destroy the SRT network,following by the largest degree and strength attack strategies.But the SRT network was robust to random failure.Besides,the decrease of function of weighted networks caused by intentional attack strategies was faster and more significant than it of topological networks,which implied that weighted networks were more fragile than topological networks when were subjected to malicious destructions.Then,a dynamic approach and process for vulnerability analysis of rail transit network was developed,which aimed at investgating the effect of dynamic characteristics(passenger flow and train flow)on network vulnerability and identifying the critical stations in the rail transit network.Based on the results of rail transit assignment and static vulnerability,we extended the concepts of betweenness in the complex network,i.e.,passenger betweenness and vehicle betweenness.By simulating different disruption scenarios,we found that the rank of passenger betweenness matched well with the rank of total time loss.Therefore,the passenger betweenness was an effective indicator to indentify potentially important nodes.However,the traditional betweenness and vehicle betweenness might not be a good indicator of node importance.As it passenger betweenness could quickly and effectively identify critical stations,and the rank of passenger betweenness was consistent with the rank of total time loss,which illustrated that complex network theory was suit for the assessment of dynamic vulnerability of the rail transit network.Finally,based on quantitative results of complex network characteristics and vulnerability analysis,we summarized the potential measures to improve the robustness of rail transit network,which could provide the scientific basis for the planning and construction of urban rail transit network.