Bottlenecks at highway or urban freeway are one of origins of traffic congestion. Based on empirical investigation on field data of Beijing urban freeway, schematic phase diagram at on-ramp vicinity is developed and detailed phase transition processes are investigated. There are five phases in the phase diagram of on-ramp vicinity in connection with on-ramp flow and upstream flow, among which are two phases only observed in urban freeway traffic. It's also found that the empirical phase diagram developed in this paper is different from previous theoretical phase diagram of highway traffic. The boundaries of each phase are influenced by traffic demand at locations, such that empirical phase diagrams are vary in quantities at different locations.Moreover, according to investigation on empirical phase transition process, coherent moving flow, as a new phase observed in urban freeway flow in our previous work, is confirmed.

At present, research on traffic flow theory has mainly focused on highway traffic, which is significantly different from urban freeway traffic. Traffic science is an "empirical science", and as such is it is based on empirical urban freeway traffic flow data. In this study, first four steady phases are identified in the flow-density plane of traffic flow: free flow, coherent-moving flow, synchronized flow and jam. Then, three modes of phase transition, i.e. spontaneous transition, propagating transition (PT) and induced transition (IT) are analyzed, and the judgment conditions as well as computable methods of PT and IT are discussed in detail. Finally, based on actual data of the Beijing urban freeway, an empirical analysis of phase transition from coherent-moving flow to synchronized flow, which is the process of "smooth traffic" transferring to "congested traffic", is presented to validate the concepts and methods proposed in this paper.

Urban freeways play an important role in urban traffic networks. Compared with highway traffic, urban freeway traffic has different characteristics, such as denser on-and off-ramps, more complex road conditions, and lower velocity limits. Until now, however, there has been no comprehensive analysis of urban traffic flow. In this paper, through an analysis of the density dependence of velocity distribution, we investigate the fundamental velocity–density relationships of urban freeways, compare them with those of highway traffic, and explain them using existing traffic flow theories.

公共交通线路调整影响分析是一个定量与定性相结合、经验与理论相结合、创新与反馈相结合的复杂过程，是一个要涉及协调多方面利益和目标的寻优过程。基于四阶段交通模型理论，使用O-D反推技术和随机用户均衡配流模型，设计了公共交通线路调整影响分析流程，达到更实际、更合理的交通分配结果。以8图层架构为基础，提出了系统总体架构，最后使用TransCAD实现系统。通过某市实际的公交路网的模拟计算，得到了合理的结果，证明了系统具有较好的应用价值。