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.