Empirical Analysis of Phase Transitions of Traffic Flow at Urban Expressway

Traffic flow features of urban ring road were analyzed based on empirical data of Beijing 2nd Ring Road.Traffic flow stability and synchronization among lanes around critical density were analyzed, ie probabilities of speed transition and deviations of traffic parameters among lanes. Four traffic phases were identified£o free flow, high-speed synchronized flow, low-speed synchronized flow and traffic jam combined with concept of synchronized flow in three phases traffic flow theory. Moreover, spatial-temporal evolvement rules of traffic flow based on these four phases were analyzed by empirical data. Results show thai traffic flow phase and spatialtemporal variation rules among phases by analysis have application value for short term prediction, control and inducement of traffic flow. Meanwhile it can provide theoretical base for analysis of traffic congestion mechanism and establishment of traffic flow model.