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1 1 Introduction This Guide presents the findings and best practices of NCHRP 03-118, âDecision-Making Guide for Traffic Signal Phasing.â It is intended to guide practitioners in selecting the most appropriate traffic signal phasing for most of the conditions traffic signal professionals typically face. It is not intended to be inclusive of all conditions. This introduction addresses the need for and purpose of the guidance, the scope of the guidance, the project development process, and how to effectively use the guidance. 1.1 Need for the Guidance Few resources provide information related to selecting the most appropriate traffic signal phasing for the various geometric and operational situations encountered in the field. This is especially true for left-turn signal phasing decisions with respect to the level of control for the left-turn movement (phase mode) and whether left turns should precede or follow the main through movement (phase sequence). Ample guidance relates to the proper implementation of different types of phasing; however, national, uniform guidance for the selection of phase mode and sequence is limited and insufficient. According to survey results collected as a part of this study and included in Appendix A, only 29% of responding agencies reported having formal policies, guidelines, or procedures for selecting the most appropriate phasing for a signalized intersection (Figure 1). Figure 1 - Breakdown of Agencies with and without Formal Policies for Signal Phasing Furthermore, traffic signal phasing practices may differ considerably from one agency to the next, creating a lack of uniformity across the nation. Various aspects of traffic signal phasing have gaps in both knowledge and consistency. Many professionals indicated a need for further design guidance on topics such as time-of-day fixed phasing/dynamic phasing and alternatives to protected-only left-turn phasing. 28.6% 53.8% 17.6% Agencies with a Formal Policy for Signal Phasing Agency Has a Formal Policy Agency Does Not Have a Formal Policy Unsure / Not Known
2 1.2 Purpose of the Guidance The purpose of this document is to give traffic engineers, designers, and transportation professionals a decision-making tool with quality information to select the most appropriate signal phasing for a given location and set of traffic conditions. The goal of this Guide is to give professionals designing or operating signalized intersections the tools they need to provide safe and efficient overall operations, considering both crash risk and movement delays. This Guide provides comprehensive, clear, and concise guidelines and direction for decisions related to traffic signal phasing and equips practitioners with the information needed to select the most appropriate traffic signal phasing. This Guide is intended to provide best practices, but it is not intended to serve as a regulatory standard or requirement. Engineering judgment should be applied throughout the traffic signal phasing selection process. 1.3 Scope The intent of this Guide is not to comprehensively cover every detail of traffic signals; instead, it is to give a review of traffic signal phasing aspects and provide guidelines for the selection of the most appropriate signal phasing approach. The following aspects of vehicular phasing at signalized intersections will be covered in this guidance: ⢠Best practices for phase mode ⢠Best practices for phase sequence ⢠Best practices for pedestrian phasing ⢠Balancing safety and operations considerations While this Guide discusses pedestrian phasing, it does not cover pedestrian phasing in its entirety with respect to safety considerations. Pedestrian phasing is further covered in a recent Federal Highway Administration (FHWA) publication (FHWA 2018) on pedestrian phasing at signalized intersections. The following items are not covered or are not covered in detail in this document: ⢠Signal timing ⢠Signal head placement ⢠Associated signing and pavement markings ⢠Intersection design ⢠Traffic signal controller equipment or operating protocol ⢠Vehicle and pedestrian detection methods ⢠Emergency Vehicle Preemption (EVP) ⢠Railroad Preemption ⢠Pedestrian Hybrid Beacons and Emergency Vehicle Hybrid Beacons (EVHBs) ⢠Unconventional intersection types such as Single-Point Intersections or Diverging Diamond Interchanges (DDI) ⢠Transit priority or preemption systems Though very important in the overall design of an intersection, these items are not directly related to the selection of traffic signal phasing and are well documented in other guidance and policy documents. Many of these items relate to the implementation of the selected phasing strategy. For more information on
3 signal timing implementation, refer to the Signal Timing Manual, 2nd Edition (NCHRP 2015). The Manual on Uniform Traffic Control Devices (MUTCD) (FHWA 2009b) and state supplements provide design standards and guidance for a variety of topics, including signal head placement, intersection signing, and pavement markings. The information presented in this Guide is primarily intended for traditional three- and four-leg signalized intersections. This Guide assumes that traffic signalization is warranted by the MUTCD (FHWA 2009b) and/or state and local requirements, and that alternative intersections have already been considered. This Guide is intended for use at existing signalized intersections or intersections to be signalized. 1.4 Guidance Development Process A literature review, included in Appendix B, was completed to identify current practices across the nation and ascertain any gaps in knowledge that need to be filled. Additionally, a survey was sent out to the Institute of Transportation Engineers (ITE) members asking both about their usage of different traffic signal phasing strategies as well as aspects of phasing that need more guidance. Professionals representing 23 states in the United States and two provinces in Canada provided responses. From these results, a research plan was created to target the items flagged as lacking in knowledge/guidance. The project team collected traffic signal, geometric, and crash data from the following agencies for use in the signal phasing analysis: - City of Bellevue, Washington - City of Chicago, Illinois - Maine Department of Transportation - City of Mesa, Arizona - Montana Department of Transportation - New Hampshire Department of Transportation - City of Peoria, Arizona - City of Phoenix, Arizona - City of San Antonio, Texas - City of Seattle, Washington - City of Scottsdale, Arizona - City of Tempe, Arizona - Wisconsin Department of Transportation This Guide synthesizes the best practices and additional information obtained from the research study efforts to provide guidance on the best traffic signal phasing selection for a variety of intersection geometric and operational features. Users of this Guide should understand the basic concepts of traffic signal phasing, some of which are defined within this Guide. Users of this Guide should also have a good understanding of traffic signal design, traffic signal timing, and Part 4 of the MUTCD, which are not included in this Guide. Following the basic concepts, more information about phasing methods are presented, including current and best practices from the literature review, ITE survey, and research efforts, to aid the traffic engineer in selecting the most appropriate phasing. After reviewing the Guide, traffic engineers will be able to use
4 their best engineering judgment to consider the necessary safety and operational factors and make an informed decision on the most appropriate signal phasing. Engineering judgment should be used in interpreting and implementing the results of this guidance. It is important to review and follow any local or state guidance in addition to this document.
