Research Of Traffic Assignment Method Of Urban Road Network Based On Cell Transmission Model

Traffic assignment is an important part of the traditional traffic planning.The purpose of traffic assignment is determining traffic loading of road network on condition that the traffic demand is known and providing basis for the traffic planning,design and management.Dynamic Traffic Assignment(DTA)has become an important branch in the field of traffic assignment.DTA can describe the changing process of traffic loading on the two dimensions of time and space.Because traffic simulation is convenient to integrate the control strategy and is able to obtain traffic data dynamically,it's used widely in DTA.Because Cell Transmission Model(CTM)is a kind of discrete simulation method based on hydrodynamics and is able to capture dynamic traffic features accurately,this article tries to apply it to the traffic assignment.1.In order to apply CTM to simulate traffic flow under the environment of urban road,iterative formula which describes the dynamic feature of cells with traffic density is deduced on account of Flow Conservation Law and Macroscopic Fundamental Diagram.Then the limitation of traditional CTM that all cells must have equal length is resolved.At the same time,cellular division method of sections at the entrance and exit as well as the intersection is proposed based on lane-use assignment and signal phase.The sending flow rate of signal-controlled cell is adjusted dynamically according to the releasing flow rate at the stop line.Then the I-CTM which considers signal control is proposed.Experiment shows that traffic density curves created by I-CTM and VISSIM fit well and description of features of queue and dissipation at a signalized intersection is close.Meanwhile,the I-CTM decreases the computation cost and consuming time.2.The simulation granularity of I-CTM is refined to individual vehicles.The function of route planning for individual vehicle is achieved.The vehicle transmission model is modified on the condition that route is known.Then the IS-CTM which considers individual vehicle is proposed.Then traffic assignment based on IS-CTM is proposed by replacing the BPR function with IS-CTM to calculate traffic impedance of section.Cases show that the results of traffic assignment methods based on IS-CTM and BPR function are close.3.Considering time-varying traffic demand and time-varying traffic state on the basis of IS-CTM,a method to evaluate traffic state of road network at different times is proposed and a set of whole simulation rule is designed including a fast algorithm for searching route,a discrete route selection model,a overall cost function of route and conditions of convergence.Then the ID-CTM applied to DTA is designed.In order to verify the feasibility of the DTA based on ID-CTM,the comparison with DTA of VISSIM was made.The experimental results show that the traffic assignment method based on ID-CTM is feasible and can be applied to the actual traffic assignment.