Research On Length Of Acceleration Lane In Left-ramp Merging Area On Urban Expressway

Urban expressway has the advantage of large capacity and fast operating speed,which plays a key role in urban road system.However,with the rapid growth of traffic volume,the ramp influence area in expressway becomes the main bottleneck of traffic operating efficiency and safety.In the form of the ramp,the right-side layout is adopted in most cases.However,due to the constraints of ground situation and construction feasibility,the form of left-side layout is also adopted in a few cases,which needs a higher requirement on the setting of traffic design parameters.Based on the traffic flow data of Nanjing Airport Expressway,this paper studies the length of acceleration lane in left-ramp merging area through the two aspects which are traffic operation efficiency and safety.Through theoretical analysis,field investigation and micro-simulation,the recommended value of left acceleration lane length is given.Firstly,the influence factors of the traffic flow operating characteristics in the merging area are analyzed qualitatively,including traffic volume,vehicle operating speed,vehicle type composition,ramp layout form and driving behavior.Then the parameters required for the investigation are defined.In the process of the investigation,the traffic flow data of Nanjing Airport Expressway in the peak period is collected by aerial photography.The traffic operating characteristics of the merging area are analyzed from the two aspects of traffic operating efficiency and safety.Secondly,the VISSIM simulation model and the SSAM conflict model are constructed according to the road parameters and the traffic flow data.The average travel delay and traffic conflicts are selected as the evaluation indexes of the traffic operation efficiency and safety in the merging area.Then the reasonable simulation times is determined.Feasibility analysis and sensitivity analysis of the default parameters are carried out.The result is selected as the parameter which needs to be corrected.Thirdly,the method of dimensionality reduction is put forward,which can calculate the sum of each index weight according to the demand,so as to make a comprehensive evaluation of the road traffic situation from the two aspects of traffic operating efficiency and safety.Then establish the correction function and determine the applicable algorithms.Through algorithms comparison,the shortcomings of the existing algorithms are analyzed.In this paper,the improved SPSA algorithm based on the fuzzy control theory is applied on parameter correction.The validity of the parameters is verified after that.Finally,the simulation experiment is designed based on the corrected parameters.The influence of the number of lanes,the length of acceleration section and the length of transition section on the traffic characteristics of merging area is explored.The suggestions for the improvement of the layout are put forward.The recommended length of left acceleration lane is also given based on the minimum average travel delay and traffic conflicts through simulation experiment,which has been verified by formula calculation.