Research On The Evolution Of Traffic Status For Grade-specific City Roads

The network of big cities in China is generally composed of four grades of roads: expressway,arterial road,sub arterial road and branch road.Due to the different functions and properties of each level of roads,the traffic flow characteristics are also quite different.For the expressway and main road around the city center,a large number of traffic flows are gathered because of both the traffic and life service functions,resulting in the network regional congestion.Therefore,based on the field data in Tianjin downtown,this paper analyzes the performance and traffic flow basic laws of different grades networks based on the Macroscopic Fundamental Diagram.Some corresponding control measures are also designed to make the traffic flow can be reasonably distributed on different levels of roads,so as to alleviate the regional congestion of network.First of all,analyzes the macro traffic status of different grade roads in Tianjin on a weekday and a weekend comparatively by Macroscopic Fundamental Diagram(MFD)model.In addition,the average speeds of different grades of roads are used as the threshold for distinguishing the traffic states,and the road traffic status of different grades of networks is analyzed combined with MFD characteristics.Some results are obtained as follows: During the peak hours of the workday,the speed of expressway fluctuates the most,the traffic flow distribution is relatively uneven,and the network is unstable.The average density of the arterial road is the largest in the peak period,the proportion of the roads with low-speed traffic flow and congested sections of the roads network is the highest.While,the average density of the sub arterial road is the lowest and the proportion of the congested road sections of the network is the lowest in each period.At the weekend,people travel relatively scattered,so that there is no obvious peak period of centralized travel.The travel traffic flow in each period is relatively uniform and the performance of road network at all levels is good.Next,this paper establishes the traffic state evolution models among each grade of networks to analyze the evolution mechanism.The model parameters are calibrated with actual data,and the sensitivity analysis is used to study the relationship between different grade road traffic states interaction mechanism.Secondly,this paper analyzes the macro traffic state of the different grades road network during the-May-holiday.The characteristics of traffic status of each grade road during the holidays are as follows: Before and after the holidays there are traffic flow peaks out of the city and back to the city respectively.The traffic flow of the expressways in and out of the city grows more than usual,and the spatial distribution is more uneven.Meanwhile,the fluctuation of the average speed and density is also greater.During the holidays,the urban traffic peak period is extended and the congestion segments are changed.So that the macro traffic state of each grade of road shows completely different characteristics from the normal traffic in scale,both in time and space.Finally,in view of the uneven distribution of traffic flow in Tianjin's road network and the resulting regional congestion,this paper designs a collaborative control model for congested areas based on the MFD control concept.The model is divided into two layers.Firstly,boundary control is performed on the congested areas to maintain the average flow of the road network in the region around the critical value.Secondly,through the collaborative optimization control of the transfer flow between the different grades road networks can further balance the traffic distribution in congested areas,so as to improve the utilization efficiency of all levels of roads.The simulation results show that the collaborative control model of different road network levels proposed in this paper can improve the traffic demand sharing capacity of sub arterial roads,reduce the traffic pressure of expressways and arterial roads,and to keep the whole network in good running state.