Empirical Investigation on Phase Diagram at Urban Freeway with On-ramp

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.