New Resource: Decision-Making Guide for Signal Phasing

Earlier this year, the TRB released NCHRP Web-Only Document 284: Decision-Making Guide for Traffic Signal Phasing. The document outlines the existing best practices on signal phasing along with the results of NCHRP Project 03-118, under which researchers developed new safety performance functions (SPFs) and crash modification factors (CMFs) for various left-turn phasing scenarios at the intersection level. The document covers guidance on left-turn phasing mode, right-turn phasing mode, phase sequence and pedestrian phasing.

Five Key Takeaways:

According to the results of a survey conducted as part of the study, only 29% of responding agencies reported having formal policies, guidelines or procedures for selecting the most appropriate phasing for a signalized intersection.

The CTDOT Traffic Control Signal Design Manual outlines the acceptable options for left-turn phasing along with some general requirements for implementation. The Decision-Making Guide for Signal Phasing provides additional guidance relating to safety and operational considerations, as well as other unique contexts, such as transitways and separated bike lanes. Various alternatives to left-turn phasing are also provided.

The Guide provides comprehensive, clear and concise guidelines for decisions related to traffic signal phasing, but it is not intended to serve as a regulatory standard or requirement.

While the Guide provides useful information for engineers to select the most appropriate signal phasing, engineering judgment should be used. It is also important to coordinate with CTDOT to ensure that all Connecticut design standards are met.

According to a literature review conducted as part of the study, no known research indicates a safety or operational benefit to matching left turn phasing mode on opposing approaches.

Some agencies require that the same left-turn phasing mode is used on opposing approaches, but in some cases, it may be more appropriate to allow permissive turns on one of the approaches. The Guide also discusses situations in which a variable mode may be appropriate.

The pedestrian phasing information presented in the Guide discusses factors to consider for concurrent vs. exclusive phasing, LPI, detection, pedestrian prohibitions, and two-phase crossings.

The information presented in the guide should prove useful for refining pedestrian crossing designs; however, it is important for the design engineer to reference the CTDOT Traffic Control Signal Design Manual and the associated Interim Pedestrian Considerations and Pedestrian Signal Design – Technical Info documents. CTDOT has clearly defined policies and standards on pedestrian signal phasing and design.

The research conducted as part of the study produced new safety performance functions and crash modification factors for left-turn phasing modes.

The Guide provides a table of crash modification factors as well as a description of a methodology for combining delay and crash costs to conduct cost-benefit analyses for various phasing alternatives.

If you have traffic signal systems questions, please contact: Theresa Schwartz, P.E.,
P.T.O.E. – Traffic Signal Circuit Rider (860) 486-4535 or theresa.schwartz@uconn.edu.

Resources:

Decision-Making Guide for Traffic Signal Phasing: http://www.trb.org/main/blurbs/181114.aspx

CTDOT Traffic Control Signal Design Manual and Related Content:
https://portal.ct.gov/DOT/Traffic-Engineering/Traffic-Control-Signal-Design-Manual