Research On Reasonable Speed Limit Of Urban Expressway

Urban expressway is an important part of the urban road, it constitutes traffic network with main trunk road, secondary trunk road and branch, servicing the urban transportation. Urban expressway is high-speed road within the city, and the growing traffic congestion issues beset the traffic operation gradually, how to improve urban expressway traffic efficiency, reduce congestion has become a problems concerned by the field of traffic. Developing a reasonable speed limit can make urban expressway’s traffic safety, efficiency and reduce traffic jam. Therefore, this paper aims to study urban expressway reasonable speed limit, and provide references for the urban expressway speed limit implementation methods.Based on traffic investigation, the flow, speed, vehicle composition and accident conditions of basic urban expressway segment are analyzed statistically, mastering distribution characteristics of urban expressway traffic flow, the discrete meaning of speed is presented, the three definitions of discrete speed description method: standard deviation of speed, adjacent car speed difference and 85% and 15% speed difference are presented and lay foundation for later analysis about the influence of discreteness of urban expressway speed to traffic accident and road traffic capacity.By analyzing the difference of following vehicle’s speed and vehicle safety distance model, get the conclusion that the greater the discrete nature, the greater the safety distance required for the vehicle. So reducing the speed divergence of urban expressway traffic can improve the safety of the vehicle. Domestic and foreign research results show that the function relationship exists between speed discreteness and traffic accident rate. The dispersion index and accident rate of function model are obtained by analyzing discrete velocity and accident rate using measured data and fitting data. The tolerance values of different discrete are determined by the tolerance values of accident rate, and the validity of the model is proved by the impact analyzing. In the aspect of the influence of velocity dispersion on the capacity, the speed of the modified coefficient model of the capacity was established, and the practical effect of velocity dispersion on the capacity was analyzed. It is concluded that under the same traffic flow density, the smaller the speed discreteness, the smaller impact on the actual capacity.The constraint model is established using the peak and peace peak periods respectively, based on the safety, comfort degree as well as the maximum capacity, the maximum speed limit model is established, the lowest speed limit of expressway is determined by considering the safety index of speed dispersion, and the recommended values of the lowest speed limit and the speed limit of the expressway under different design speeds are presented. At last, the implementation strategy of the variable speed limit for the basic road section is given, and a feasible scheme for the implementation of the speed limit is proposed.