The National Academies

NCHRP 03-54 [Final]

Uniform Traffic Signal Displays for Protected/Permissive Left Turn Control

  Project Data
Funds: $100,000
Research Agency: Kittelson & Associates, Inc.
Principal Investigator: William A. Kloos and Daniel Fambro
Effective Date: 1/9/1995
Completion Date: 12/31/1996

The growing levels of traffic have led to the development of innovative means to control traffic. Protected/permissive (P/P) traffic controls have been developed to increase the left turn capacity and reduce delay at intersections by providing an exclusive turn phase for left turns as well as a phase during which left turns can be made as traffic will allow. P/P left turn traffic controls have been implemented in a variety of ways, because the Manual of Uniform Traffic Control Devices (MUTCD) initially provided only limited guidance. The FHWA recommends a shielded, 5-section signal, tied to the adjacent through lanes for P/P left turn control. Many states have adopted the 5-section, doghouse signal located between the through and turning lanes, which provides a green arrow for the protected mode and a circular green to indicate the permissive phase. At least four variations of the display to indicate the permissive phase are known to exist, including the flashing circular red, the flashing circular yellow, the flashing red arrow, and the flashing yellow arrow. Other variations also exist in the phasing, signal displays, arrangement, signal placement, and use of supplemental signs.

Problems with P/P left turn traffic control have been identified but not been resolved. The yellow trap occurs when yellow is presented to the left turning motorist whose permissive left turn phase is terminating. To avoid the hazardous yellow trap situation using standard MUTCD displays, simultaneous lagging left turns have been used. This precludes the use of lagging operation with phase overlaps, and thus has the potential to decrease capacity and increase delay. An innovative operation known as the Dallas phasing permits phase overlaps and may eliminate the potentially unsafe yellow trap situation by allowing a continued permissive left turn during the opposite approach lagging protected left turn phase. Research has shown that this operation reduces delay and improves safety. There is need for research to address the yellow trap and driver confusion problems. In addressing these problems, it is necessary to consider the safety implications of increased signal efficiency and the difficulty in establishing uniformity between states.

The objectives of this project were to (1) review past research and current practices related to P/P left turn controls, and (2) formulate a plan for evaluating the various displays to determine the most effective. To accomplish the objectives, the following tasks were envisioned: (1) Critically review the results of NCHRP Synthesis Topic 25-03, Left Turn Treatments at Intersections to identify the nature, extent, and issues associated with the current usage of P/P left turn control treatments. (2) Define safety and operational performance factors for locations with P/P left turn control to allow assessment of the yellow trap, driver confusion, and other problems. Establish a matrix for comparing the different forms of P/P left turn control across the factors to aid in developing recommendations for uniform signal displays. (3) Identify the research necessary to address the issues associated with P/P left turn controls and determine the most viable experimental approaches e.g., driving simulators, field studies, accident analyses, quantitative analysis). (4) Develop detailed experimental plans for the specific elements of the research plan to determine which subset of P/P left turn control treatments are the safest and most effective. These experimental plans should indicate the sample sizes, potential field-study locations, sampling strategies, research methodologies, analytical techniques, screening procedures, and expected results. (5) Prepare a draft final report that documents the background studies and research plan.

Status: The draft final report has been approved by the panel and the recommended revisions were incorporated in the continuation phase under Project 3-54(2).

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