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Open Access Publications from the University of California

Scholarly Works (4 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2011)

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%.

    Cover page: Queue Interactions on an Urban Freeway and Their Influence on Discharge Flow
    • Book
    Research Reports (2015)
    Most special-use freeway lanes in the US, whether reserved for carpools, toll-paying commuters or both, are physically separated from the adjacent regular-use lanes by some form of barrier. Vehicle movements in and out of a special lane of this type are permitted only at select access points along the route. The barrier at each select point might open for a distance of 400 m or so. Limiting access in this way is said to reduce the “turbulence” that might otherwise occur were the special lane not to have a buffer, such that vehicles could instead enter or exit that lane anywhere along its length. Yet, real freeway traffic studied in spatiotemporal fashion shows that access points are prone to become bottlenecks. The problem occurs when traffic in the regular lanes becomes dense, as commonly happens during a rush. Drivers then seek refuge in the special lane in greater numbers. Since the vehicular maneuvers through the access point are focused within a limited physical space, they can become disruptive and further degrade traffic. Degradation can occur both in the special lane and in the adjacent regular ones. The damage can be worse than what occurs when barrier are not used to limit special-lane ingress and egress. Policy implications are discussed.
      Cover page: Spatiotemporal Studies of Traffic Phenomenon on Freeways with Limited-access Special Lanes
      • Article
      Research Reports (2010)
      This report furnishes clear evidence that on-ramp metering can increase the output flow through a freeway, and by so doing diminish the total time that commuters collectively spend traveling on the freeway and its on-ramps. Empirical study was performed on a 6.3-mile stretch of northbound Interstate 5 in Sacramento. The stretch spans the interchanges of Pocket Road (to the south) to W street (See Figure 1). Traffic data, both from loop detectors and from videos, were collected during the morning rush periods over a period spanning several years. Data were initially collected in 2006 prior to the deployment of ramp meters at the site. Data were collected again in 2007 and 2008 after meters were installed on five on-ramps. (The meters operate using a control logic developed by Caltrans.) Finally, a metering logic was developed in response to certain traffic details observed at the site, and was tested there in spring and fall 2009. A number of interesting and useful findings resulted from all this, as described below.
        Cover page: On-Ramp Metering and Commuter Delay: A Before and After Study
        • Article
        • Peer Reviewed
        Research Reports (2012)
        This report evaluates the performance of Continuous Risk Profile (CRP) compared with the Sliding Window Method (SWM) and Peak Searching (PS) methods. These three network screening methods all require the same inputs: traffic collision data and Safety Performance Functions (SPFs), however, depending on how these input parameters are analyzed at the network screening level, the result of the analysis can vary significantly. Findings indicated that the CRP method produced far fewer false positives than SWM and PS. The false negative rates for CRP, SWM and PS were comparable. These findings indicate that by using the CRP method, California Department of Transportation (Caltrans) can significantly reduce the resources spent on investigating falsely identified locations and better utilize the resources in improving high collision concentration locations. It will also help Caltrans in reducing the backlog in Caltrans Table C.
          Cover page: Experimental Evaluation of the Continuous Risk Profile (CRP) Approach to the Current Caltrans Methodology for High Collision Concentration Location Identification
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