Research On Determination Method Of Speed Limits Of Multilane Freeway

Many freeways in China have been extended or reconstructed to satisfy the increasing traffic demand,as a result,the mileage of freeways which number of lanes is six or more than six increases year by year.Vehicles have greater degree of driving freedom in multilane freeways than in two-way four-lane freeways,and the mutual disturbances among mixed flow bring about the loss of operating efficiency and the occurrence of safety problems.Changes of the facilities also provide possibilities for diversified management measures.Determining speed limits scientifically and reasonably has practical significance to improve operating efficiency and safety of multilane freeways.The research can provide reference for decision makers.Multilane freeways with integral subgrade are taken as the research object.Firstly,traffic characteristics of multilane freeways are discussed.According to the non-parametric test of operating speed on freeway,vehicles are divided into four types.Data collected by video observation is used to analyze the traffic characteristics in multilane freeways,including features of traffic volume,velocity,headway and crashes.The characteristics of the speed limit methods of the multi-lane freeway are analyzed and a framework for determining the speed limit is constructed.Relationships among design speed,expected speed,operating speed and limit speed are discussed,and influences of speed on traffic safety are illustrated.The paper compares features of different speed limit methods of multilane freeways.Following clarifying the purposes and influential factors of speed limit,a two-step determination framework for speed limit value of multilane freeway is proposed,which distinguishes the mainline speed limit value from speed limit value of each lane.Take 85% operating speed as the main basis,the method of determining the maximum speed limit of the mainline based on engineering technology is put forward.Taking the differences of driving behavior of different vehicle type into account,a cellular automata model for multilane freeway traffic flow is constructed.The speed standard deviation,speed difference between the front and rear vehicles,the lane change rate are selected as measures of operating safety,while the operating speed for operating efficiency.Values of these indicators are output data of the model.The actual survey data is used to calibrate parameters in the model and also to verify the validity of the model.The adaptability of the different lanes function division schemes of multilane freeway is explored by using the constructed simulation model.The purposes and principles of multilane freeway lane partitioning are analyzed.The characteristics of different lane partitioning strategies are qualitatively compared.The operating efficiency and safety level in conditions of different lane division schemes are quantitatively discussed,where the influences of traffic volume and truck ratio are studied.The applicable conditions of each lane partition scheme are summarized.Based on the analysis of the lane partition scheme,the determination method of lanedivided speed limit value is put forward,which aims to optimize the operating safety and efficiency collaboratively.Traffic efficiency,speed standard deviation,and lane change rate are used as indicators,and the comprehensive evaluation method based entropy weight is employed.Combining with the simulation model,the implementation effects of the different lane-divided speed limit schemes for six-lane and eight-lane freeways under the scenarios of low traffic volume,moderate traffic volume and high traffic volume are respectively studied,and the recommended solutions of speed limits of each lane under various scenarios are given.Some relevant management suggestions are also proposed.