Control Method Of Variable Speed Limits For Freeway Under Foggy Weather Conditions

With the rapid increase of China’s freeway mileage and vehicle amount,freeway safety situation has become increasingly severe,serious threat to lives and property of people.Foggy as a common weather,leading to the drivers to be hesitating and illusory due to reduced visibility,which has negative impact on freeway’s traffic safety and efficiency.Posted speed limit has some limitations for that cannot to be dynamical adjusted according to the real-time visibility distance and traffic states.The variable speed limit(VSL)as a burgeoning traffic control technique,estimates the real-time traffic states by advanced traffic detection technique,and adjusts the speed limits according to some control strategies,to improve the traffic states.Presenting a VSL control method for freeway under foggy weather conditions is beneficial to ensure traffic safety and efficiency.This study tries to analyze the actual measured traffic data for highway in the fog,and establish the dynamic traffic flow model based on research that traffic parameters effected by fog.Second,analyzes the impacts of VSL control on the traffic states and establish a VSL model under foggy weather condition basis on driver compliance and VSL input.Then,presenting a VSL control scheme with system design,workflow and VSL operations strategy that based on Model Predictive Control.Finally,evaluation and complete of the control model and control methods by using simulation software VISSIM.Main works are as follows:(1)Getting the traffic data under foggy weather condition on the basis of field research on highway,after a series data processing,analyze the traffic parameters such as vehicle speed,density,and headway effected by visibility distance on the both traffic safety and efficiency.This result provides a data support for research on the highway traffic state in fog and propose the traffic flow model.(2)Proposed a dynamic traffic flow model under foggy weather condition based on classic Cell Transmission Model,and consider the degree of traffic parameters reduction in fog and the bottleneck capacity drop,the actual measured traffic data are used to calibrate the weather adjustment factor and model.By means of simulations verifies the traffic flow model through MATLAB software.It turned out that the traffic flow model can make it up to accurately simulate and forecast the actual traffic states,and provides a basis of model for VSL model and method.(3)Analysis the traffic safety,efficiency and traffic flow fundamental diagram effected by VSL,and proposing a dynamic traffic flow model under VSL in fog based on considering driver compliance and VSL input.Proposed a VSL control scheme with system composition,workflow and VSL operations strategy,in particular,the VSL operations strategy based on Model Predictive Control,and set up the target function and constraint conditions by selecting the traffic conflicts from traffic safety,electing the total travel time and total travel distance from efficiency.This method can use to support of highway traffic management.(4)Based on the traffic simulation software VISSIM,using Java programming language to achieve contrastive simulation with control model and operations strategy,and evaluate result from traffic safety and efficiency.The results of simulation show that the proposed VSL control method can increase the lowest speed,decrease the traffic conflicts and the maximum speed difference of adjacent links,ensure traffic safety and efficiency for freeway under foggy weather conditions.This study can provide data support and a basis of model for research the highway traffic state under foggy weather condition,it also can use to the further highway traffic management.Another fact-due to the time variation and complexity of traffic state,weather and traffic data should collect widely from different regions and highways,and considering the cooperative control of mainline and ramp,for providing the further research on highway traffic state and VSL control method under foggy weather condition.