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Open Access Publications from the University of California

Traffic Interactions in Freeways with Carpool Lanes

  • Author(s): Jang, Ki Tae
  • Advisor(s): Cassidy, Michael J
  • et al.

The dissertation presents findings in regard to the vehicular interactions that arise when carpools and regular vehicles are segregated in their own lanes. Empirical evidence shows that the presence of a carpool lane can diminish vehicular lane-changing maneuvers near bottlenecks, which in turn can increase bottleneck discharge flows in adjacent lanes. This so-called smoothing effect is so pronounced that in many cases even an underutilized carpool lane can increase total bottleneck discharge flows and therefore benefit all freeway commuters and not only carpoolers. Ironically, the congested regular-use lanes are often damaging to the carpool-lane travelers.

It is found that slow speeds in a carpool lane can be due to both, high demand for that lane and slow speeds in the adjacent regular-use lane. These findings imply that the current US policy to restrict most classes of Low-Emitting Vehicles (LEVs) from slow-moving carpool lanes can be counterproductive. The LEVs excluded from carpool lanes will thereafter contribute to congestion and slowing in the regular lanes, and this, in turn, can also reduce the speeds of those vehicles that continue to use the carpool lanes, despite the reduced use of those lanes.

Implications for all of the above findings are discussed. This includes discussion on constructive ways to amend the new regulation, and promising strategies to increase the vehicle speeds in carpool lanes by improving the travel conditions in regular lanes.

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