Modeling And Simulation Of The Traffic Flow In Urban Freeway Based On Cell Automata

Although the construction of urban expressways has enjoyed rapid development in recent years, the speed is slower than traffic growth, and at present there is no uniform standard for expressway construction in China. Besides that, in the constructed expressway system, many problems exist such as the ramp control means are alone, the management efficiency is low and so on. So further study is needed to the expressway system.This paper mainly pays attention on one of the bottlenecks which influence the natural exertion of expressways—the on-ramps and off-ramps. Firstly, based on the measured data, this paper analyzes the traffic flow characteristics of the on-ramp and off-ramp and the impact of the ramps to the upstream and downstream of the main line in the three-lane expressway. This provides the real basis for the foundation of the model. Then, a three-lane expressway cell automata model which is more accordant to practice is built under different circumstances after some revisions to the lane-changing rules based on the cell automata theory and the early study on the traffic flow cell automata.In the study of the on-ramp control, modeling and simulation are done under three situations:no control in the on-ramp, stop to let others go, ALINEA control. Through the comparation of the simulation results, we find the effect of the ALINEA control is better; After that, to the observed phenomenon that the impact of the first on-ramp to the main line is stronger than the second on-ramp when there are two series on-ramps in the expressway, the ALNEA control has its own dificiency; At last, this paper builds the three-lane expressway model which has two continuous on-ramps through the introduction of HMI, the results reveal that the transition of traffic flow states of the three lanes at the point between the two ramps is non-homogeneous, and the HMI changes abruptly, while the transition of traffic flow states of the three lanes at the point after the second ramp is homogeneous and the HMI does not change abruptly, so this paper gets and verifies the conclusion that whether the HMI changes abruptly should be one of the conditions which to decide whether to implement the ALINEA control to the on-ramp.This paper also constructs the cell automata models for a segment of the multi-lane expressway near off-ramp region which includes the decelerating lane or not respectively, the lane-changing times and lane utilization ratio are calculated and compared with the factual data to verify the model, after the following comparation of the simulation results under the two circumstances, the paper finds that when the off-ramp is connected with the expressway through the decerating lane, the efficiency of the expressway system is enhanced; To the quantitative analysis of the length of the decelerating lane, the models are built considering whether there is the on-ramp connecting with the upstream of the expressway, the results reveal that the length of the decelerating lane is not the longer, the better, in fact the proper length is relevant with the traffic flow, when there is on-ramp in the upstream, the length of the decelerating lane should be longer than the one when there is no on-ramp in the upstream; At last, a simple traffic flow guidance is introduced to study whether it can improve the expressway system's efficiency near the decelerating lane.