Some observed features of traffic flow phase transition at urban expressway diverge bottlenecks

Recurring bottleneck sections are the direct cause of congestion in urban expressways and diverge bottlenecks can generate a more adverse impact on mainline flow. In this paper, two diverge bottlenecks were investigated using data collected from multiple days. The main findings are: (1) Relationship between Pre-Queue Flow (PQF) and Queue Discharge Flow (QDF) are related to diverge ratios. (2) Breakdown is triggered at the lane or lane group with the highest occupancy. (3) As soon as the occupancy of the initial lane or lane group rises to near 30%, the congestion begins to spread laterally to the rest of the lanes. (4) Different congestion propagation modes in the longitudinal direction are found using the spatiotemporal speed contour diagrams. (5) The recovery from congestion flow to free flow is the inverse process of that from free flow to congestion flow. These findings provided a detailed description of traffic flow phase transition that can be considered as a useful reference for theoretical studies and can help engineers to design traffic management schemes.