The Coordination Method Of Expressway Speed Limits And Ramp Control In The Fog

In foggy days,the vehicle congestion and traffic safety of highway has been an urgent problem that needs to be solved,especially those chain collisions caused by heavy fog.In view of this problem,many domestic and foreign scholars have launched a lot of research.They mainly use single mainline speed limit control or separate on-ramp control,but the lack in considering the both controls for expressway simultaneously.Therefore,this paper takes the operational efficiency and vehicle safety as the main purpose,establishes a ramp control and mainline speed limit control model of highway,and proves the effectiveness of the model.Firstly,this paper takes Jiujiang Loop Expressway and Changsha Ring Expressway as examples to collect traffic data,which mainly collects the foggy and sunny day's data,and then,summarizes the traffic flow characteristics of foggy weather.Secondly,according to the traffic characteristics of the fog,the expressway traffic flow model of foggy day is established.Considering the influence of ramp control on mainline density,this paper establishes a mainline density function,increases the impact of ramp control rate on mainline density.Regarding the effect of mainline speed limit on driving speed,the expected speed of traffic flow model is modified.At the same time,with a view to the impact of low visibility in foggy day on driving speed,the function between speed and visibility is built.After revising the expressway traffic flow model of normal weather,the foggy day's traffic flow model of highway can be get.Furthermore,considering the fog and visibility,ramp control rate,vehicle queue length on ramp and other factors,taking the mainline operational efficiency,queue length on ramp and traffic safety as the objective function,regarding road density,queue length on ramp and speed as the decision variable,considering the mainline density restriction,the mainline velocity constraint,on-ramp queue restriction,the mainline capacity constraint and other constraints as constraint conditions,this paper builds a multi-objective mixed-integer programming model.And then it uses the Elitist Non-dominated Sorting Genetic Algorithm(NSGA-II)to get the optimization solutions of model.Finally,this paper analyzes and evaluates those four different schemes from mainline efficiency,front distance,space headway and driving safety in terms of these four aspects.It builds an expressway model by using micro-simulation software VISSIM.After comparing and analyzing the benefit of this model with the other three contrasts,the result shows that when the saturation is less than 0.7,this model can get the optimum benefit;when the saturation is more than 0.7,the average travel time of this model is higher than those comparison schemes.The average travel time increased 3%,but the driving safety indicators improved 12.23%.It proved that the model in high-saturated traffic situation could still guarantee the safety requirements.