Elise Miller-Hooks

Modeling Violations in High-Occupancy Toll Lane Studies

Project 22

To mitigate congestion along freeways, managed lanes, e.g. high occupancy vehicle (HOV) or high occupancy toll (HOT) lanes, operating concurrently with general purpose (GP) lanes have gained popularity across the nation. Among the construction options to separate managed and GP lanes, non-barrier separation techniques, which use only solid pavement markings, are increasingly employed. These techniques inform drivers that crossing between GP and managed lanes is prohibited; however, they permit nearly unlimited improper ingress/egress to/from the managed lanes. Even with significant enforcement, violation rates related to non-barrier separated managed lanes in the U.S. are considerable. In fact, the national average annual managed lane violation rate, which includes both occupancy- and access-type violations, was estimated in 2005 to involve between 10 and 15 percent of all vehicles using managed lanes. These violations negatively impact mobility, safety and revenue. Despite this, no prior model developed for the purpose of predicting improvements in traffic performance metrics and the potential revenue that can be raised through the introduction of a new HOT lane facility within an existing roadway or to assess potential practicable operational strategies and facility designs has incorporated this violation behavior. Nor has any prior study systematically considered the impact of violation on the performance of these facilities or quantified the impact of violations on safety. This study sought to assess the importance of these omissions.

In this study, the potential impact of violations related to HOT lane access and vehicle occupancy on traffic performance in managed and GP lanes was quantified for an existing roadway segment with single HOV lane and proposed HOT lane facility conversion. Techniques were developed for modeling violation behavior in concurrent flow lane operations within a widely used microscopic traffic simulation tool. The significance of the violation impact on traffic performance for future managed flow lane facility performance and benefit analyses was assessed in extensive and systematically designed experiments.

This study also investigated and quantified the safety impact of access-type violations. Specifically, it was hypothesized that violations, particularly those pertaining to managed lane egress and ingress, lead to sudden changes in speed of approaching vehicles. These sudden changes in speed can propagate upstream, further resulting in congestion and increased speed variance (or traffic instability) over the affected portion of the roadway. A three-step simulation-based methodology was developed to quantify the impact on safety as a consequence of increased speed variation and changes in congestion. In the methodology, safety is measured by the length of discontinuities in traffic speed resulting directly from violation incidents as determined through inspection of traffic speed contour maps. The larger the total length of discontinuities in the traffic speed contour map, the greater the speed variability and the less safe the situation is presumed to be. In addition to safety implications of increased speed variability, increased congestion may result as a consequence of a sudden decrease in vehicular speeds. Under certain levels of traffic flow, as congestion increases, interactions among vehicles increase, and there may be secondary safety effects.

Award Period:
Aug. 2008 - Aug. 2009
Source of Funding:
Tier 1 University Transportation Center, United Stated Department of Transportation
PI on subaward (co-PI on main contract)
Total Award Amount:

Project 22


Elise Miller-Hooks, Ph.D.
Bill & Eleanor Hazel Chair in Infrastructure Engineering

Phone: 703.993.1685
Email: miller@gmu.edu

Office: 4614 Nguyen Engineering Building

Sid and Reva Dewberry Department of Civil, Environmental and Infrastructure Engineering
George Mason University
4400 University Drive, MS 6C1
Fairfax, VA 22030


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