Flow processes at a freeway bottleneck.

Flow processes in the vicinity of a high-volume fixed bottleneck on a metered freeway were studied by comparing detector data with video tapes. Evidence is presented to support the hypothesis that maximum flow rates decrease when queues form. In addition, despite volumes significantly in excess of accepted values of capacity downstream of the on-ramp, the normal point of queue formation was about 1,500ft upstream of the merge area. Queues appear to start in the leftmost lane, which carries extremely high volumes at this point. Shock waves resulting from obvious conflicts in the merge area are rare and normally occur only after the establishment of the queue upstream, despite 6-min merge rates of about 2,500 to 2,800 veh/hr.