Protected/permissive (P/P) left turn traffic controls 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 opposing traffic will allow. The protected left turn can either precede (lead) or follow (lag) the opposing through signal phase. 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 limited guidance. At least six displays to indicate the permissive phase are known to exist (i.e., straight five-section head, five-section "doghouse" head, flashing circular red, flashing circular yellow, flashing red arrow, flashing yellow arrow). Variations also exist in the phasing, signal displays, arrangement, signal placement, and use of supplemental signs. There have been concerns that some of these variations may confuse motorists, and validation of their relative operational and safety advantages is needed.
A key concern with P/P left turn traffic control is the "yellow trap" which occurs during the change from permitted left turns in both directions to a lagging protected left turn in one direction. The MUTCD requires that a permissive left turn operate simultaneously with the adjacent through movement. The left turning driver whose permitted interval is ending may try to sneak through the intersection on the yellow indication, not realizing that the opposing through traffic still has a green indication. To avoid the yellow trap, most agencies do not use leading/lagging P/P left turns. An innovation known as the "Dallas phasing" allows this operation without the yellow trap by operating the permissive left turns simultaneously with the opposing through movement. Research has shown that this operation reduces delay and improves safety, but it is not easily implemented in all situations.
The objective of this project was to evaluate the safety and effectiveness of different signal displays and phasing for protected/permissive (P/P) left turn control through laboratory and field studies and to recommend uniform signal display(s) for P/P turn control. The research considered both leading and lagging operations associated with P/P control, other situations where the "yellow trap" occurs, and typical intersection configurations.
To accomplish the project objective, the following tasks are envisioned: (1) Form an advisory group to assist in evaluating P/P left turn signal displays. (2) Develop an evaluation matrix and conduct an engineering assessment of the different displays. (3) Survey drivers to determine understanding of the various displays using static and dynamic displays. (4) Conduct traffic operations studies to determine and compare traffic conflict rates for the various displays. (5) Review crash data to compare left-turn crash rates associated with the various displays. (6) Conduct a comprehensive analysis of the results from the preceding tasks. (7) Prepare a draft version of a video driver survey to be used in Task 10. (8) Prepare an interim report which documents the findings of the first six tasks. (9) Participate in a meeting with the panel and advisory group to review the interim report and the plans for the Task 10 efforts. (10) For the most promising displays, confirm the results of earlier tasks related to driver understanding of the displays using driver simulators and field evaluations of the flashing yellow arrow. The driving simulator tests will evaluate the green ball in a cluster configuration, a green ball in a separate signal head using Dallas phasing, and a flashing yellow arrow in a separate head. (11) Prepare a second interim report which documents the findings of preceding tasks. (12) Participate in a second interim meeting with the panel and advisory group to review the second interim report, project recommendations, and implementation plan. (13) Prepare a final report documenting the research study methods, their associated findings, and the results of the panel meetings. The report will recommend a standard P/P left turn display with supporting justification and an implementation plan.
The project is complete. The research team presented the results to the National Committee on Uniform Traffic Control Devices in January 2003. Some questions were raised at that meeting that were investigated using auxiliary NCHRP funds. Further presentations were made at the June 2003 and June 2004 meetings of the NCUTCD.
Product Availability: NCHRP Report 493, Evaluation of Traffic Signal Displays for Protected/Permissive Left-Turn Control
, is available. The report includes CRP-CD-35, NCHRP Report 493
Supplementary Material: Working Papers
. The contractor has established a website at http://projects.kittelson.com/pplt/index.html
, where additional information on the project can be found.