Study On Reasonable Intervals Of Optimal Traffic Performance Index Of Road Network Efficiency

With the acceleration of urbanization and motorization,traffic congestion is becoming more and more serious.In order to evaluate the traffic performance state of road network scientifically and objectively and grasp the development trend of congestion accurately,a series of traffic evaluation indexes including traffic performance index are put forward.Traffic performance index is a quantitative measurement of traffic performance status of the conceptual value,has been widely used in traffic control and strategic planning.Many cities have established a traffic performance index monitoring platform to monitor the road network all day and release it in real time.However,there is a lack of in-depth research on the reasonable value of traffic performance index and its reasonable range.In view of this,this paper takes the traffic performance index based on the mileage ratio of severe congestion currently used in Beijing as an example to study the theoretical optimal traffic performance index and the reasonable intervals of the optimal traffic performance index.First of all,on the basis of summarizing the traffic fundamental diagram model and its application,this study seeks the theoretical optimal efficiency state of road network based on the traffic fundamental diagram model,and defines the theoretical optimal efficiency state of road network when the traffic flow reaches the capacity.At this point,the corresponding traffic performance index,namely the traffic performance index corresponding to the critical speed,is the theoretical optimal traffic performance index.Secondly,this study uses floating car data,RTMS and traffic survey data to establish the traffic fundamental diagram model of Beijing.It is found that Van Aerde model has high adaptability and good fitting effect for expressway traffic flow.The critical speeds of expressway,arterial road and minor arterial road are 34.60km/h,22.93km/h and 18.70km/h,respectively.The critical speed of each grade of road is the key parameter to calculate the theoretical optimal traffic performance index.Thirdly,by analyzing the relationship between road network average speed and traffic performance index of each grade,the paper establishes the road network average speed-traffic performance index model,and based on this model,establishes the calculation method of theoretical optimal traffic performance index of each grade road and the whole road network.The theoretical optimal traffic performance indexes of expressway,arterial road and minor arterial road in Beijing are calculated to be 4.16,6.14 and 6.17 respectively.The theoretical optimal traffic performance index of the whole road network is 5.54.Since this index corresponds to the state when the road network is close to the capacity,there is a risk of breakdown.Therefore,in the actual traffic management,it is necessary to intervene in advance to control the traffic performance index not higher than the theoretical optimal value,so as to avoid strictly reaching the theoretical optimal efficiency state.Then,this dissertation considers the breakdown to study the reasonable intervals of the optimal traffic performance index.Based on the analysis of the characteristics of the speed and the speed difference of the expressway,the threshold method is used to identify the breakdown,and the breakdown probability models of the expressway in different periods are established.Then,the numerical relationship model of road network speed of each grade is established,which is used to calculate the reasonable intervals of the optimal traffic performance index under different breakdown probability.Traffic managers can select the acceptable breakdown probability range and the corresponding reasonable interval according to the risk tolerance of breakdown and the characteristics of urban traffic flow.Taking Xicheng District and Chaoyang District as case studies,it is found that Chaoyang District has a lower breakdown probability than Xicheng District,and maintains a longer time within reasonable intervals of different breakdown probability ranges than Xicheng District,which indicates that the performance and stability of Chaoyang District’s road network is higher,and also indicates that the reasonable intervals of the optimal traffic performance index can also reflect the performance of the road network to a certain extent.