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The Smoothing Effect of Carpool Lanes on Freeway Bottlenecks

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  • Cassidy, Michael J
  • Jang, Kitae
  • Daganzo, Carlos F

Abstract

Real data show that reserving a lane for carpools on congested freeways induces a smoothing effect that is characterized by significantly higher bottleneck discharge flows (capacities) in adjacent lanes. The effect arises because disruptive vehicle lane changing diminishes in the presence of a carpool lane. The effect is reproducible across days and freeway sites: it was observed, without exception, in all cases tested. Queueing analysis shows that the effect greatly reduces the times spent by people and vehicles in queues. By ignoring the smoothing effect at one of the sites we analyzed, for example, one would predict that its carpool lane increased both the people-hours and the vehicle-hours traveled by well over 300%; when in reality the carpool lane and its attendant smoothing reduced both measures. The effect is so significant, in fact, that even a severely underused carpool lane can in some instances increase a freeway bottleneck’s total discharge flow. This happens for the site we analyzed when carpool demand is as low as 1200 vph. It follows that strategies designed to induce smoothing by other means also hold promise for managing congestion, both for freeways that have carpool lanes and those that do not. Possible strategies of this kind are discussed.

Suggested Citation

  • Cassidy, Michael J & Jang, Kitae & Daganzo, Carlos F, 2008. "The Smoothing Effect of Carpool Lanes on Freeway Bottlenecks," Institute of Transportation Studies, Research Reports, Working Papers, Proceedings qt6fk4s29c, Institute of Transportation Studies, UC Berkeley.
    • Handle: RePEc:cdl:itsrrp:qt6fk4s29c
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    1. Newell, G. F., 1993. "A simplified theory of kinematic waves in highway traffic, part II: Queueing at freeway bottlenecks," Transportation Research Part B: Methodological, Elsevier, vol. 27(4), pages 289-303, August.
    2. Munoz, Juan Carlos & Daganzo, Carlos F, 2002. "Fingerprinting Traffic From Static Freeway Sensors," University of California Transportation Center, Working Papers qt1mf4n2w8, University of California Transportation Center.
    3. Cassidy, Michael J. & Rudjanakanoknad, Jittichai, 2005. "Increasing the capacity of an isolated merge by metering its on-ramp," Transportation Research Part B: Methodological, Elsevier, vol. 39(10), pages 896-913, December.
    4. Laval, Jorge A. & Daganzo, Carlos F., 2006. "Lane-changing in traffic streams," Transportation Research Part B: Methodological, Elsevier, vol. 40(3), pages 251-264, March.
    5. Newell, G. F., 1993. "A simplified theory of kinematic waves in highway traffic, part I: General theory," Transportation Research Part B: Methodological, Elsevier, vol. 27(4), pages 281-287, August.
    6. Cassidy, Michael J. & Daganzo, Carlos F. & Jang, Kitae & Chung, Koohong, 2006. "Empirical Reassessment of Traffic Operations: Freeway Bottlenecks and the Case for HOV Lanes," Institute of Transportation Studies, Research Reports, Working Papers, Proceedings qt31h8z81t, Institute of Transportation Studies, UC Berkeley.
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    Cited by:

    1. Eric J. Gonzales & Nikolas Geroliminis & Michael J. Cassidy & Carlos F. Daganzo, 2010. "On the allocation of city space to multiple transport modes," Transportation Planning and Technology, Taylor & Francis Journals, vol. 33(8), pages 643-656, September.

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