Layout Structure Characteristics Of Urban Multimodal Public Transit Network

The public transport,with its advantages of large transport capacity,high efficiency and low energy consumption,has become an important means to alleviate urban traffic congestion and ensure its sustainable development.After years of construction,the public transit network has become more and more mature in major cities generally and is developing towards a multimodal direction gradually.Focusing on its integrated supply performance,the research on the layout structure characteristics of urban multimodal public transit network is carried out in order to promote the improvement of the collaborative planning theory for urban public transport,provide scientific basis for the design,construction and operation management of public transport and promote the reasonable investment of construction funds and the full play of coupling efficiency.Considering the flexibility of passenger carrying tool and the characteristics of operating route,a four-level classification system for urban public transport is proposed.The supply characteristics of each transit mode are analyzed.The connotation of urban multimodal public transport system and public transit network is elaborated and the research scope of this paper is defined.The urban public transit network is proposed to be divided into three layers of backbone network,trunk network and auxiliary network.The development motivation for urban multimodal public transit network is analyzed from two aspects of policy orientation and travel demand,meanwhile,its development constraints are discussed from the perspective of economy,population,land use and roads.Based on that,the development trend of the layout structure is summarized,including multimodal,multilayered,integrated,open,reliable and stable,fair and equitable.Considering the differences in the routes of two-way transit lines and the public transport capacity of different transit modes,the augmented Space L network model for the expression of the topology of urban multimodal public transit network is proposed,whose modeling method is elaborated and graph model as well as matrix representation are established.Focusing on the functions of network elements,such as line connection and transport capacity turnover,the characteristic indexes of the model are constructed.Taking the public transit network of Nanjing as an example,the model is builded and the topology characteristics are studied.The results show that:Both out-degree and in-degree of the nodes follow the exponential distribution.The odd and even degree distribution of the nodes present a property of dual exponential distribution indicating that there is a "Matthew effect" in the development of the public transit network.The cumulative frequency of the node strength follows the exponential distribution,which means that the transport capacity of the public transit network has a property of hierarchical structure.The regular bus stations still play an important role in serving the connection of transit travels,while the rail transit stations play an important role in serving the turnover of public transport capacity in the context of the integration of multiple networks.The average shortest distance and the diameter of the multimodal public transit network are 15.2 and 59 respectively,which are 10.6%and 6.3%lower than those of the trunk network.The loading of the backbone network strengthens the connectivity and the small-world effect of the network.Xuanwu Avenue is the most important in improving the efficiency of transit travel.Drawing on the definition of the urban passenger terminal,considering the external function of connection and the potential function of contact,the connotation of hub station is defined and the definition of hub node is improved accordingly.In order to characterize the hub function,the Hub index is proposed.The calculation model for Hub index is established adopting the method of grey correlation analysis based on variation coefficient weighting considering degree,strength,PageRank,weighted PageRank and betweenness comprehensively.The method for station classification and hub identification based on Hub index and cluster analysis with fixed classes is proposed.Taking the public transit network of Nanjing as an example,the Hub index of each station is calculated and the stations are classified accordingly.The results show that:There are 5 first-class hubs in total,which are located in the center of each group in the city.Most of them possess the function of connecting the external traffic of the city,which are the core of the public transit network within the research scope.There are 9 second-class hubs,most of which are located near commercial areas,scenic spots and large residential areas,playing an important role in one or more aspects of servicing line connection,passenger distribution and inter-regional turnover of passenger flow.There are 57 third-class hubs,which take Xinjiekou-Daxinggong as the core and radiate along the metro lines and the inter-regional traffic arterials undertaking the function of transit travel turnover within the area.The connotation of the vulnerability and the robustness of urban multimodal public transit network is defined from two aspects of reality and abstraction.The attack test is introduced.Considering the network connectivity and the travel efficiency,the selection principle of robustness measurement index is proposed.Based on that,a measurement index system is established.The one-by-one attack mode is introduced and two types of attack strategies,local and global,are designed.The procedure of network attack test is designed and its implementation program is written based on Python.A case study on the public transit network of Nanjing is carried out.Based on local attack,it is pointed out that a total of 168 stations and 167 inter-station sections are the vulnerabilities to maintain network connectivity.Based on global random attack,it is pointed out that after 40 and 35 rounds of consecutive average,the robustness of the network against random attacks on stations and inter-station sections has become stable so that the characteristic line of network robustness is constructed accordingly.Based on global deliberate attack,it is pointed out that:The network is relatively less robust to the deliberate attacks on stations based on degree and PageRank as well as the deliberate attacks on inter-station sections based on betweenness.Relatively,the deliberate attack on stations is more destructive to the network.In addition,under different requirements or constraints,the order of the network robustness against each attack is different.From the perspective of social economics,the connotation of the layout equilibrium of public transit network is defined,whose research idea is proposed.For the road layout of public transit network,the P-R bipartite network model is established.The matrix representation and characteristic indexes are constructed from two aspects of the route and the transport capacity.The equilibrium measurement idea combining Gini coefficient and Atkinson index is proposed.The general analytic formula of the Lorenz curve for the layout equilibrium analysis of public transit network is constructed so that the calculation method for Gini coefficient is optimized.The recommended value of equilibrium preference parameter is given,namely ε=1.5 so that the optimized calculation method for Atkinson index is established.Taking the public transit network of Nanjing as an example,the spatial geographic information database based on ArcGIS is established and the dependence characteristics of the public transit network on roads are analyzed.The Gini coefficient and the Atkinson index corresponding to each characteristic index are obtained.It is pointed out that:The overall layout structure of Nanjing public transit network is inequitable.The allocation of the line and its utilization of road space are relatively equitable and reasonable,but the allocation and the distribution in the urban road network of public transport capacity are slightly inequitable.The realization of the efficiency and the accessibility of the PTN are highly dependent on a small number of stations and road sections.In generall,when ε=1.5,there is no obvious difference between the grading results obtained by Atkinson index and Gini coefficient,but there is a certain discrimination between either of the specific measurement values that the former has a higher sensitivity,which proves the effectiveness and complementarity of those in the equilibrium measurement of public transit network.