Safety Evaluation And Modeling For Exits Of Urban Expressway Using Conflicts Theory

Urban expressway is an important part of urban roadway system.Urban expressways were constructed in large cities recently to alleviate traffic congestion.Due to the lack of regulation of guidance signs in related standard,accidents tend to happen in the exit and entrance area of urban expressway.Moreover,the accidents rate of exit area is much higher than trance area.Accident on urban expressway would lead to congestion in a large scale.Therefore,it is necessary to study the safety of exits of urban expressway.There is enough achievement on traffic conflicts modeling,traffic flow of urban expressway and safety analysis of highway from abroad and domestic studies.Most of studies focus on ramps on interchange of freeways.However,few researches studied of safety of urban expressway.This paper aims to explore the method to evaluate the safety of urban expressway.Urban expressway in Xi'an is studied in this paper.Five locations including one of the 2ⁿdd South ring road four of 3^rd ring road were chosen to carried out investigations.200m adjacent area of upstream is defined influence area of exit ramps.Unmanned plane and video were combined to collect data on sites.Software such as Tracker and VIP/T were applied to process video.Dataset including traffic volumes,speed,density,headway,gaps,accelerate speed and traffic composition.Operating speed is under 80km/h for expressway and traffic composition is under 25%,which indicates that there are fewer large vehicles appearing on urban expressway than on highways.Speed of inner lane is higher than the outer lane.First of TTC?time to collision?is used to evaluate the traffic safety,which is calculated by headway,length of vehicle and speed of vehicles.Association analysis indicated that TTC was closely related to traffic volumes,traffic composition and speed deviation.The value of TTC of inner lane is larger than outer lane.This paper defined concepts of individual risks and social system risks.The relative indicators were also used to evaluate the safety of urban expressway.The social system risks defined as the risks of whole system,which is related to traffic volume,speed deviation and traffic composition.Individual risks is used to describe the feeling of individual drivers and it is related to features of surrounding vehicles.The Mixture Gauss Model is used to approximated the real distribution of TTC.In this study,three Gauss compositions were estimated by EM method to achieve the final the mixture Gauss model.In order to determine the threshold of TTC to determine conflict,TTC was set to2s,3s,4s,5s,respectively.The proportion of conflicts was calculated for 5 locations.It was found that Location 1 is the worst among the five locations.The linear regression models for individual risks and social system risks were developed at the end.0.9 R² value coefficient indicated models fits the real data well.The results provide a method to evaluated the safety of urban expressway.