Capacity Change Mechanism of a Diverge Bottleneck

This paper offers a more complete picture of the capacity change mechanism at a diverge bottleneck caused by a queue from an exit ramp moving upstream onto the freeway. Detailed observations collected by videos during a 4-day period from an expressway in Bangkok, Thailand, showed that traffic states during bottleneck activation at the diverge site could be categorized into three distinct states with different levels of capacity according to vehicle speeds on through movement lanes and the rates of lane change maneuvers near the off-ramp queue. The data indicated that the transitions of traffic states were caused primarily by the changes in exit flows. The lower capacity was initiated by a more restrictive off-ramp flow that caused some cut-through vehicles on the adjacent lane to impede through movement traffic. Once the exit flow increased, impeding exit vehicles could move out of the adjacent through lane and higher capacity could be restored. These findings point to automatic off-ramp control strategies that would generate higher bottleneck capacities through detecting traffic speeds on freeway through lanes. These findings advance the present knowledge in traffic flow theory that will enable traffic researchers to understand more fully the traffic phenomena at a diverge bottleneck and will help traffic engineers to operate freeway traffic properly.