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Queue Interactions on an Urban Freeway and Their Influence on Discharge Flow

  • Author(s): Kim, Kwangho
  • Advisor(s): Cassidy, Michael J..
  • et al.
Abstract

This thesis unveils a reason for the time-varying patterns in discharge flow that are commonly observed at freeway bottlenecks. Observations reveal that four well-known effects in freeway traffic can interact upstream of a bottleneck in ways that trigger periodic bursts in its discharge flow. A hypothesis concerning the capacity-increasing mechanism is that following an initial "capacity drop", (i) the bottleneck's expanding queue can trigger a new and more restrictive bottleneck further upstream. As a result, (ii) the queue downstream recedes toward the original bottleneck, leaving free flow conditions in its wake. If the tail of this receding queue (iii) passes an on-ramp junction that resides upstream of the original bottleneck, then (iv) a higher bottleneck discharge flow can temporarily ensue. We also hypothesize that this higher discharge rate should diminish after some minutes due to the capacity drop phenomenon. The process would then start anew. Detailed data collected from a 3-km freeway stretch over multiple days support this hypothesis.

The above suggests that the flow through the bottleneck can be increased by metering the freeway's on-ramps in a coordinated but unconventional way. Experiments with this approach consistently produced long-run bottleneck discharge flows that were higher than those generated from a more traditional metering policy. The average difference over multiple days was nearly 3%.

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