At present, researches on traffic flow theory mainly focus on highway traffic, which is significantly different from urban freeway. Because traffic science is an "empirical science", it is based on actual traffic flow data of urban freeway, the statistical features of velocity distribution at different densities have been investigated in this paper. Moreover, four steady phases: free flow, coherent-moving flow, synchronized flow, and jams are identified in the flow rate-density plane of the traffic flow. Finally, features of the four phases as well as functional forms of phase boundaries are discussed in detail.