| Urban expressway is an important part of the urban road traffic system.It has the characteristics of large flow,fast speed,and entrance and exit control.It carries the motor vehicle travel with the fast speed and long distance.In recent years,with the rapid growth of urban traffic demand,serious traffic congestions have occurred on urban expressways,leading to a lot of travel delays and traffic accidents.The merging zone,as the most common type of expressway bottleneck,is the area where the main-line traffic flow meets the onramp merging flow.The serious conflicts between vehicles trigger frequent traffic problems such as congestion and accidents,then result in the significant phenomenon called "capacity drop".It is shown that the flow discharge rate of the bottleneck is lower than the regular capacity.In this study,taking the ramp junction bottleneck area on the elevated entrance of Yingtian Street in Nanjing as an example,UAV aerial photography and video image processing technology are used to get high-precision vehicle trajectory data.Combined with the traffic congestion characteristics of the merging bottleneck,macroscopic and microscopic research for explaining capacity drop is carried out.The research focuses on analyzing the characteristics of spatial and temporal evolution of the decline in discharging rate,and understand in depth the impacts of microscopic behaviors such as lane changing and car following during the evolution process of capacity drop.First of all,the process of vehicle trajectory extraction and denoising in the merging area should be clarified.After selecting a typical case of the merging bottleneck,UAV aerial photography and image recognition technology are used for vehicle recognition and tracking before and after congestion in the study area.Considering the threshold values of speed and acceleration,cubic exponential smoothing method is used to smooth and modify the trajectories for obtaining complete and continuous vehicle trajectory data.The procedure of trajectory extraction and data processing would guarantee the reliable data for the subsequent research.Secondly,it is necessary to clarify the characteristics of traffic flow before and after the decline in capacity.According to the previous study on the definition of the capacity drop,the research area is divided into several regions,and the oblique cumulative curves of important cross sections are carried out identify the moment of capacity drop in the merging area.Based on traffic flow theory,the time-varying characteristics of traffic flow and vehicle operating characteristics in this bottleneck area are analyzed to study the correlation between traffic flow parameters and capacity drop.This research prepares for the evolution of the capacity drop evolution process in vehicle trajectory characteristics.Thirdly,the relationship between the evolution of the decline in capacity and the vehicle trajectory should be figured out.By constructing the oblique cumulative curves of the continuous section in the main lane,the characteristic points of the continuous capacity drop are extracted.Combined with the spatiotemporal feature maps from the three-phase traffic flow theory,the connection between the capacity drop evolution process and the traffic phases is constructed,and the trajectory characteristics of each capacity stage are analyzed.Finally,Grey Correlation analysis and Newell car-following model are used for extracting microscopic characteristic parameters such as lane-changing and car-following behaviors,and a microscopic lanechanging impact model is constructed to analyze the disturbance propagation characteristics of the rear vehicle from lane-changing,and quantifies microscopic features of capacity drop.Combined with the Newell-Daganzo model,a simple analysis model of capacity drop is built for further analyzing the cause of the discharge rate reduction in the bottleneck area.Then,a microscopic capacity calculation model based on driving behaviors is constructed. |
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