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12 3 Traffic Signal Phasing Selection Process 3.1 Traffic Signal Phasing Selection Steps This section of the Guide is intended to outline the steps in the traffic signal phasing selection process. Additional details on each step are provided in subsequent chapters. Figure 6 - Traffic Signal Phasing Selection Process Step 1: Understand the Need The need may be the installation of a new traffic signal or change in phase of an existing or rebuilt traffic signal. The need for this process may stem from the intersectionâs crash history, operational issues, or public concerns. This process may also be used as a planning-level effort on a corridor rather than in response to a need at a specific intersection. This process may also verify phasing at an existing signalized intersection. Whatever the motivation for examining the signal phasing, it is important to keep the need in mind throughout the process. This Guide assumes that a signalized intersection is the existing condition and/or the preferred alternative. Additional alternatives, such as alternative intersections or movement restrictions, should be considered prior to starting this process. Step 8 - Monitor and Maintain Step 7 - Implement and Observe Step 6 - Document Evaluation and Decisions Step 5 - Determine Intersection Operation Step 4 - Intersection Evaluation Left-Turn Phasing Right-Turn Phasing Pedestrian Phasing Sequence Step 3 - Data Collection Geometric Safety Operations Volume Step 2 - Understand the Context Step 1 - Understand the Need
13 Step 2: Understand the Context The operating context of a traffic signal extends beyond the physical location of the intersection and includes additional considerations such as user and movement priorities, state and local agency goals, and corridor context. User and Movement Priorities Users of the intersection include drivers of vehicles, including trucks and transit vehicles, pedestrians, and bicyclists. The prioritization of users and movements may vary from intersection to intersection. For example, pedestrians may be prioritized higher at an intersection in an urban area adjacent to a school than at a rural intersection with no surrounding pedestrian generators. Additionally, through movements on a suburban commuter corridor may be prioritized over low-volume side street movements. It is important to understand that all the needs of all users must be balanced, and one user should not be prioritized to the extent that it degrades basic accommodations for any other users. State and Local Agency Goals This Guide applies throughout the U.S., but preferences between different state and local agencies may vary. Throughout the traffic signal phase selection process, the user must consider state and local preferences beyond those stated in this Guide. Each state and local agency may also choose to set specific mobility and safety goals, which should be considered for a successful selection. Users of this Guide shall follow all state and local regulations and standards. Corridor Context The corridor context includes considerations such as the area type (i.e., urban, suburban, or rural), if the traffic signal will operate in coordination with nearby traffic signals, if the traffic signal will operate in isolation, the characteristics of nearby traffic signals, adjacent land uses, and general corridor traffic patterns. Step 3: Data Collection For existing intersections, the following items should be field-measured where feasible. Where field measurement it not feasible, they may be obtained from the most recent as-built plans. For new intersections, these parameters must be obtained from design plans. Agencies may have specific processes for measuring and/or documenting these parameters. a. Geometric The following geometric data should be collected for each approach to the intersection: - The number of lanes and lane use configuration - Number and length of left-turn and right-turn storage lanes - Characteristics of departing lanes - Presence of left-turn lane offset - Available sight distance for a left-turn - Crossing distance for through and left-turn movements - Geometric ability of opposing left turns to operate concurrently - Pedestrian crossing distances
14 b. Safety The traffic signal phase selection process relies heavily on crash data, which is defined differently for existing and new intersections, as discussed below. Existing Intersections The previous three to five years of crash history should be obtained for existing intersections. Five years of crash history is preferred, especially for lower-volume intersections and with respect to pedestrian or bicyclist crashes, but three years of crash history will typically suffice for higher-volume / higher-crash locations. Note if any major changes have occurred within the previous three to five years, and only consider those years consistent with existing conditions, particularly if a traffic signal has been installed or a change in traffic signal phasing has occurred. For example, if a traffic signal was installed four years ago, crash data should only be considered from the date of the traffic signal installation and not before that date when the intersection was unsignalized. Engineering judgment should be used to determine if a change is substantial enough to affect the safety analysis portion of the process. Crash History Analysis Each crash should be categorized as potentially correctable or uncorrectable by protected-only phasing. The crash analysis throughout this Guide focuses primarily on left-turn opposing crashes; however, pedestrian and bicyclist crashes (with left-turning motorists) should also be used in the analysis as applicable. Hard-copy crash reports with descriptions and diagrams will provide a more comprehensive picture of the safety characteristics compared to tabular crash data. New Intersections For new intersections, or intersections where crash history is otherwise unavailable, annual crashes may be estimated based on safety performance measures. Refer to Chapters 9 and 11 for additional information on the crash estimation process. c. Operations The speed limit or 85th percentile speed on each approach should be collected. For new signals, the current phasing strategies of existing surrounding signalized intersections should be noted. For existing signals, the phasing (mode and sequence) of the existing signalized intersection and existing surrounding signalized intersections should be recorded. Existing intersection design aspects (e.g., available equipment and existing traffic signal displays) may also be helpful. Lastly, the 85th percentile, or operating, speed of each approach may be used in the selection process. If the 85th percentile speed is not available, the operating speed may be estimated at the speed limit plus 7 mph (NCHRP 2012). At some intersections, microsimulation analysis may be helpful in signal phase selection. In these cases, operational information required for a simulation analysis should be collected as available and in compliance with any local and state guidance for simulation model development. d. Volumes Two types of volume data are useful in the traffic signal phase selection process: Average Annual Daily Traffic (AADT) for each intersecting roadway, and turning movement counts for the intersection.
15 For existing intersections, the traffic volume data may be obtained from existing data sources (e.g., spreadsheets or geographic information system (GIS) data maintained by a state or local agency) that contain volume counts no more than three years old, or the data may be obtained through field collection. Existing traffic volume data greater than three years old may be used in areas with steady traffic patterns where the older data still represents existing conditions. Many agencies and planning organizations maintain databases of accepted AADT counts; however, turning movement counts often require field collection. For new intersections, or for future analysis, traffic volume may be estimated using standard volume estimation and trip generation procedures. Turning movement counts should ideally include a classification by vehicle type, pedestrians, and bicyclists. In most cases, at least morning and afternoon peak hour, turning movement counts should be collected on a typical weekday. At some intersections, counts at other times, such as midday (noon hour), weekend, school dismissal, or factory shift change, may be helpful. If the start and end times of the peak hours are unknown, it may be necessary to collect data for more than one hour during each period to identify the peak hour. Step 4: Intersection Evaluation The next step in the process is to determine the most appropriate phasing strategies using the information provided in the guidance. Phasing for the left-turn, right-turn, and pedestrian movements should be chosen simultaneously as the decision for one movement affects the decisions for the other movements. a. Determine most appropriate left-turn phasing strategy (mode and sequence) Chapters 4 and 6 cover left-turn phasing mode and sequence concepts. The safety and operational considerations for selecting the most appropriate left-turn phasing are presented in Chapters 8-10. b. Determine most appropriate right-turn phasing strategy (mode and sequence) Chapters 5 and 6 cover right-turn phasing mode and sequence concepts. The safety and operational considerations presented in Chapters 8-10 may be applied to right-turn phasing; however, CMFs and SPFs for right-turn phasing are not provided. c. Determine most appropriate pedestrian phasing strategy (mode and sequence) Chapter 7 discusses pedestrian phasing selection in detail. Step 5: Determine Intersection Characteristics After determining the traffic signal phase mode and sequence, it will be necessary to determine additional intersection characteristics, such as the need for actuation by approach or individual movement, cycle length, timing, coordination, and signal head design. This step may not be necessary if evaluating an existing signal where the outcome recommends maintaining the existing signal phasing. In this case, the additional intersection characteristics have already been determined and are in operation, but the user may reassess these if desired. Chapter 8 touches upon these subjects, and other references, such as the FHWA MUTCD (FHWA 2009b) and the Traffic Signal Timing Manual, 2nd edition (NCHRP 2015), provide detailed information.
16 Step 6: Document Evaluation and Decisions Decisions and reasoning about signal phasing should be recorded and maintained for future reference. All existing intersection data and conditions should be documented and kept as well. Documentation should comply with an individual agencyâs process. Step 7: Implement and Observe The next step is to implement the chosen parameters. After implementation, field observations should be performed along with data collection of key operational and safety metrics. Final timing adjustments may be made as needed during the field implementation process. Step 8: Monitor and Maintain The traffic signal phase selection process is an ongoing process and requires ongoing monitoring and maintenance after implementation. Monitoring and maintenance helps to ensure that the most appropriate phasing strategy is in place even as conditions surrounding the intersection (e.g., adjacent development) change. When operating conditions become undesirable, such as increased crashes or excessive delay, the intersection may benefit from an alternative phasing strategy. At that time, the user should revisit this process, starting at Step 1, to determine the most appropriate traffic signal phasing. 3.2 Guidance Application Engineering judgment must be applied throughout the entire process. The guidance provided in this document does not cover the characteristics of every type of intersection or operation and is not a fixed set of instruction; rather, it provides general guidance that can be applied to specific intersections. This Guide is intended to be concise; therefore, the chapters reference multiple other documents and policies for additional details. The guidance provided in this document is intended to complement the additional reference sources and is not intended to conflict with, replicate, or replace any of the referenced documents or other national, state, and local policy documents. The user of this Guide is responsible for consulting and following the most current versions of each of the referenced documents, as well as any applicable national, state, and local policy.
