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Abstract
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[SafetyLit note: The the ITE approval of revised guidelines for yellow change and red clearance intervals vindicates the theory of Mats Järlström, who was fined $500 by the Oregon State Board of Examiners for Engineering and Land Surveying for publicly criticizing traffic light timing without first obtaining a professional engineering license.
The ITE’s vote updates a 55-year-old equation with Järlström's formula, which takes into account the time drivers need to slow down when making a turn in an intersection.]
The report provides a description of the recommended methods to calculate traffic signal change and clearance intervals. The calculation methodology in the report is based on the extended kinematic equation and is shown in both U.S. and metric units. The report provides guidance for applying the methodology and for selecting input values for both through and turning movements at signalized intersections. Input values include perception-reaction time, approach speed, deceleration rate, approach grade, intersection width, vehicle length, and conflicting movement start-up delay. The report notes application techniques for wide intersections and bicycle traffic. The application of measures of effectiveness and recommendations for monitoring and evaluation close the report.
What's New
The final recommended practice was updated in response to comments received on the proposed version of the document and the guidance provided by the panel convened to adjudicate appeals on the report. As a result there a number of new or revised elements in comparison to previous practice documents and previous versions of the proposed recommended practice as follows.
- The recommended calculation method uses an extended kinematic equation formula as the basis for calculation of change intervals, which is documented in the literature review (Jay Beeber (M) provides an explanation of Mats Järlström's derivation of the extended kinematic equation on page 34). Text and formula references in the guidance emphasize that the calculation of the yellow interval is the minimum value.
- With the use of the extended kinematic equation, the discussion of application to left turning movements has been expanded to include intersection entry velocity and an allowance for the use of longer change intervals for these movements of up to maximum value of seven seconds. In addition, guidance is provided for several typical signal phasing methods.
- Since there is limited research on the complex nature of driver behavior, interactions, and theoretical formulation for right-turn maneuvers, some elements of these factors are not fully understood. Therefore, more information is necessary before making a definitive, separate recommendation for change and clearance intervals for right-turning vehicles.
- Inclusion of a simple grade factor in the extended kinematic equation formulation for non-zero grades.
- Supplemental discussion and guidance related to determining intersection width.
- Modification of the discussion on the use of conflicting movement start-up delay and removing the requirement for the use of non-zero value.
- Use of measured primary data is preferred wherever possible.
- An approach for estimating values of approach speeds is offered for when the primary 85th percentile speed data is unavailable.
Background
With the importance of this topic and the amount of study devoted to it, a consensus has been difficult to reach over the years on the most appropriate method of timing the yellow change and red clearance intervals at traffic signals. ITE published a proposed recommended practice in 1985 titled Determining Vehicle Change Intervals that was not ratified by the ITE International Board of Direction to become a recommended practice.2 Nine years later, ITE published an informa- tional report titled Determining Vehicle Signal Change and Clearance Intervals.3 In 2001 ITE published another informational report, A History of the Yellow and All-Red Intervals for Traffic Signals, summarizing the development of practice up to that year.
In the interim, changes in technology, automated enforcement, the availability of new primary data, further research, as well as the public and professional concern that a defined standard of reference did not exist with regard to this topic, have led to the initiative to develop this report. Conversations between ITE leaders and the Federal Highway Administration identified specific guidance on engineering methods for traffic signal change and clearance intervals as a gap in engineering practice in the period. This took place immediately prior the October 2007 release of the request for proposals for the National Cooperative Highway Research Program (NCHRP) project that would become NCHRP Report 731: Guidelines for Timing Yellow and All-Red Intervals at Signalized Intersections.5 However, ITE's development process for recommended practices follows a different development model than NCRHP projects, and includes peer review, a public comment period on the proposed recommended practice, and an appeals process.
ITE began the initial work drafting a recommended practice with launch of the NCHRP project in 2008. An initial draft of the report was completed prior to the release of NCHRP 731 in 2012. Subsequently, the recommend practice was completely revised by the volunteer technical committee, as well as a round of review panel comments leading to the release of the proposed recommended practice in February 2015. The technical committee, with ITE staff support, worked through addressing the public comments with detailed responses to each commenter and the review panel completed another evaluation on the resulting document. As result of this input and hundreds of individual comments from the technical committee, review panel, and public across multiple drafts, the recommended practice was reviewed and responses prepared. In September 2018, ITE issued a Notice of Intent to Adopt the recommended practice, which was appealed. The technical committee—again working with ITE staff—prepared responses to the appeals, the technical committee made changes where there was agreement, and ITE issued a second Notice of Intent Adopt, that was also appealed. This led to the convening of an Appeals Panel on August 28, 2019 and the ensuing guidance providing direction for concluding the technical revisions to the final version of Guidelines for Determining Traffic Signal Change and Clearance Intervals Recommended Practice ...
Purpose and Intended Use While municipal, county, and state jurisdictions have defined practices or procedures on the determination of change and clearance intervals at signalized intersections, historically there has been a lack of consensus best practices available in the United States and Canada. The guidelines are based not only upon existing information found during the initial research, but also on the collective experience of ITE staff, committee members, peer review panel, and others who participated in the development process.
ITE's intent for the proposed recommended practice is to reflect a thoughtful balance between sound engineering theory and practical application. The recommendations presented in the report should yield reasonable times for the yellow change and red clearance intervals for traffic signals. These will allow the profession to balance those durations while enhancing intersection safety, maintaining reasonable traffic flow, and providing for movement of vehicles, bicycles, and pedestrians. The goal of the recommended practice is to create a consensus methodology for calculating and evaluating traffic signal change intervals that can be uniformly and consistently implemented by transportation agencies.
This report should not supersede engineering judgment. It is anticipated this document will be updated periodically to refine the procedures based on experiences of agencies using it and studies performed by the research community. Note that this report is specifically focused on the timing of traffic signal change intervals and does not discuss or intend to discuss pedestrian signal change intervals. |