Analysis On The Capacity Of The Secondary Separation Exit Ramp At Freeway Interchanges

The total mileage of highways in China has been increasing rapidly,and interchanges at highway interchanges have become more and more common.At the interchanges of highways,heterogeneous traffic flows separate,weave,and merge,which contributes to different types of vehicles changing lanes and mixing in this area.Interchanges can be divided into main line,diverge junction,ramp,and merge junction.The off-ramp influence area is not only a bottleneck prone to congestion,but also a dangerous section prone to accidents.This paper selects a special type of off-ramp: the secondary separation exit ramp at highway interchanges as the research object.This research conducted a detailed and comprehensive study on the following contents,including: sub-types divided by configuration,road geometric characteristics,micro and macro traffic flow characteristics,the flow of outside key lanes,the capacity values of main lines and outside key lanes,and the improvement method of the capacity.Based on the map survey,according to the intersection angle of the interchange and the connection method of the ramp,this study subdivided the secondary separation ramp into six sub-types.A number of locations containing research objects at home and abroad were selected.PEMS detector data were collected at foreign locations,and trajectory data were analyzed through UAV aerial video for domestic locations.Domestic data that could not be obtained was supplemented by simulation experiments.This research first defines the research scope,components,and subdivision configuration of the secondary separated exit ramp.Secondly,the macro characteristics of the exit ramp,such as the relationship between flow and speed,trajectory diagram,lane change position and lane change rate;and micro characteristics,such as the speed,acceleration,deceleration,head distance and their distributions are analyzed quantitatively and concretely.The SUMO simulation experiment was used as a method to supplement data.On the basis of verifying its accuracy,the flow relationship in each direction of the secondary separation exit ramp was analyzed.The outside key lane flow estimation model is proposed.Learning from HCM2016 practice on the vehicle operation status of the exit ramp,the research team explores the influence of various factors,such as flow of each section,on the flow of outside key lanes of secondary separation exit ramps.Then,the flow estimation models of the outside key lanes are established respectively for six-lane,eight-lane and ten-lane freeways,and the prediction accuracy is compared with the accuracy of the HCM2016 model.The results elaborate that the applicability of the model is quite good,and it reflects the linear relationship between the rightmost key lane flow and the mainline flow,as well as the logarithmic relationship between the outside key lane flow and the right/left-turn flow.The maximum traffic volume will be obtained from multiple research locations through simulation experiments,which means,the main-line capacity(pcu/h)and outside key lane capacity(pcu/h).Combining the capacity values obtained from the simulation experiment with the theoretical capacity values derived from the model,the respective capacity values of the main-line and outside key lanes of the secondary separation exit ramp will be proposed in this study(using the number of lanes as the dividing standard).Taking the actual domestic secondary separation exit ramp as an example,according to domestic design specifications,the unreasonable lane layout and the length of the deceleration lane can be changed respectively to improve the capacity of the secondary separation exit ramp.The results show that the layout and length of the deceleration lanes at two separations both have a significant influence of the traffic capacity in the diverge area.The applicability of the secondary separation exit ramp is also closely related to the whole linear design,as well as the intersection angle.