Study Of Variable Speed Limit Control At Bottlenecks Of Freeway Mainline

The highway traffic bottlenecks are usually formed due to road constructions or traffic accidents. Setting a speed limit value at a certain distance upstream of the bottleneck to keep the dissipation rate near the maximum traffic capacity, variable speed limit (VSL) can not only reduce the congestion and the length of queue, but also can increase the passage efficiency and traffic safety. On the contrary, the limited speed will cause big local speed difference between upstream and downstream of the control section, thus the rate of traffic risk will increase greatly. Therefore, in order to ensure the vehicle slowing down their speed under the condition of safely gradient speed, this paper established a control method of gradient velocity dropping on the basis of a single speed limit strategy.Firstly, this paper analyzed the applicability condition of the traffic flow model and used the METANET traffic flow model as the basic traffic flow model of variable speed control. In order to make the model more suitable for traffic flow characteristics on highway bottlenecks,the paper improved the model in the pilot response time and the parameters were calibrated by some relevant data. Then, on the basis of control principle and process of variable speed limit, this paper used the total time of passage and the total distance as the optimization objective function and established the variable speed limit control strategy by adopting the method of predictive control frame model. Based on the actual traffic demand, this paper used MATLAB simulation method to assess the feasibility of the control strategy. The simulation results showed that the variable speed limit control method can obviously reduce the total vehicles' transit time to 10.8% and improve the road traffic service level within the effective control range.On the basis of analyzing the characteristics of the driver, this paper established the relationship between the value of speed dropping down and the collision rate of the road section by the VISSIM traffic simulation method. The simulation results show that the collision rate is minimized when the vehicle speed decreases between 35km/h to 45km/h.Based on the road safety speed constraint, the speed limit strategy of bottleneck section is established, and the method of gradient velocity value and series division have been introduced by this paper. Taking the standard of construction section, the calculation method of each level's length is analyzed with the three phase- speed limit.