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Signal Timing Manual - Second Edition (2015)

Chapter: Chapter 5 - Introduction to Timing Plans

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Suggested Citation:"Chapter 5 - Introduction to Timing Plans ." National Academies of Sciences, Engineering, and Medicine. 2015. Signal Timing Manual - Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22097.
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Suggested Citation:"Chapter 5 - Introduction to Timing Plans ." National Academies of Sciences, Engineering, and Medicine. 2015. Signal Timing Manual - Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22097.
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Suggested Citation:"Chapter 5 - Introduction to Timing Plans ." National Academies of Sciences, Engineering, and Medicine. 2015. Signal Timing Manual - Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22097.
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Suggested Citation:"Chapter 5 - Introduction to Timing Plans ." National Academies of Sciences, Engineering, and Medicine. 2015. Signal Timing Manual - Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22097.
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Suggested Citation:"Chapter 5 - Introduction to Timing Plans ." National Academies of Sciences, Engineering, and Medicine. 2015. Signal Timing Manual - Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22097.
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Suggested Citation:"Chapter 5 - Introduction to Timing Plans ." National Academies of Sciences, Engineering, and Medicine. 2015. Signal Timing Manual - Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22097.
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Suggested Citation:"Chapter 5 - Introduction to Timing Plans ." National Academies of Sciences, Engineering, and Medicine. 2015. Signal Timing Manual - Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22097.
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Suggested Citation:"Chapter 5 - Introduction to Timing Plans ." National Academies of Sciences, Engineering, and Medicine. 2015. Signal Timing Manual - Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22097.
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Suggested Citation:"Chapter 5 - Introduction to Timing Plans ." National Academies of Sciences, Engineering, and Medicine. 2015. Signal Timing Manual - Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22097.
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Suggested Citation:"Chapter 5 - Introduction to Timing Plans ." National Academies of Sciences, Engineering, and Medicine. 2015. Signal Timing Manual - Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22097.
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Suggested Citation:"Chapter 5 - Introduction to Timing Plans ." National Academies of Sciences, Engineering, and Medicine. 2015. Signal Timing Manual - Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22097.
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Suggested Citation:"Chapter 5 - Introduction to Timing Plans ." National Academies of Sciences, Engineering, and Medicine. 2015. Signal Timing Manual - Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22097.
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Suggested Citation:"Chapter 5 - Introduction to Timing Plans ." National Academies of Sciences, Engineering, and Medicine. 2015. Signal Timing Manual - Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22097.
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Suggested Citation:"Chapter 5 - Introduction to Timing Plans ." National Academies of Sciences, Engineering, and Medicine. 2015. Signal Timing Manual - Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22097.
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Suggested Citation:"Chapter 5 - Introduction to Timing Plans ." National Academies of Sciences, Engineering, and Medicine. 2015. Signal Timing Manual - Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22097.
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Suggested Citation:"Chapter 5 - Introduction to Timing Plans ." National Academies of Sciences, Engineering, and Medicine. 2015. Signal Timing Manual - Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22097.
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Suggested Citation:"Chapter 5 - Introduction to Timing Plans ." National Academies of Sciences, Engineering, and Medicine. 2015. Signal Timing Manual - Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22097.
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Suggested Citation:"Chapter 5 - Introduction to Timing Plans ." National Academies of Sciences, Engineering, and Medicine. 2015. Signal Timing Manual - Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22097.
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Suggested Citation:"Chapter 5 - Introduction to Timing Plans ." National Academies of Sciences, Engineering, and Medicine. 2015. Signal Timing Manual - Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22097.
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Suggested Citation:"Chapter 5 - Introduction to Timing Plans ." National Academies of Sciences, Engineering, and Medicine. 2015. Signal Timing Manual - Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22097.
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Suggested Citation:"Chapter 5 - Introduction to Timing Plans ." National Academies of Sciences, Engineering, and Medicine. 2015. Signal Timing Manual - Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22097.
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Suggested Citation:"Chapter 5 - Introduction to Timing Plans ." National Academies of Sciences, Engineering, and Medicine. 2015. Signal Timing Manual - Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22097.
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Suggested Citation:"Chapter 5 - Introduction to Timing Plans ." National Academies of Sciences, Engineering, and Medicine. 2015. Signal Timing Manual - Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22097.
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Suggested Citation:"Chapter 5 - Introduction to Timing Plans ." National Academies of Sciences, Engineering, and Medicine. 2015. Signal Timing Manual - Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22097.
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Suggested Citation:"Chapter 5 - Introduction to Timing Plans ." National Academies of Sciences, Engineering, and Medicine. 2015. Signal Timing Manual - Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22097.
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Suggested Citation:"Chapter 5 - Introduction to Timing Plans ." National Academies of Sciences, Engineering, and Medicine. 2015. Signal Timing Manual - Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22097.
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Suggested Citation:"Chapter 5 - Introduction to Timing Plans ." National Academies of Sciences, Engineering, and Medicine. 2015. Signal Timing Manual - Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22097.
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Suggested Citation:"Chapter 5 - Introduction to Timing Plans ." National Academies of Sciences, Engineering, and Medicine. 2015. Signal Timing Manual - Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22097.
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Suggested Citation:"Chapter 5 - Introduction to Timing Plans ." National Academies of Sciences, Engineering, and Medicine. 2015. Signal Timing Manual - Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22097.
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Suggested Citation:"Chapter 5 - Introduction to Timing Plans ." National Academies of Sciences, Engineering, and Medicine. 2015. Signal Timing Manual - Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22097.
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Suggested Citation:"Chapter 5 - Introduction to Timing Plans ." National Academies of Sciences, Engineering, and Medicine. 2015. Signal Timing Manual - Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22097.
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Suggested Citation:"Chapter 5 - Introduction to Timing Plans ." National Academies of Sciences, Engineering, and Medicine. 2015. Signal Timing Manual - Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22097.
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Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Chapter 5. Introducon to Timing Plans CHAPTER 5 INTRODUCTION TO TIMING PLANS CONTENTS 5.1 BASIC SIGNAL TIMING CONCEPTS ................................................................................... 5-1 5.1.1 Movement and Phase Numbering ...................................................................................... 5-1 5.1.2 Ring-and-Barrier Concept ..................................................................................................... 5-4 5.1.3 Left-Turn Phasing ..................................................................................................................... 5-5 5.1.4 Overlaps ...................................................................................................................................... 5-11 5.1.5 Detector Assignments ........................................................................................................... 5-16 5.1.6 Load Switch Assignments .................................................................................................... 5-18 5.2 CRITICAL MOVEMENT ANALYSIS ................................................................................... 5-19 5.2.1 Step 1: Record Demand Volumes ..................................................................................... 5-20 5.2.2 Step 2: Determine Critical Phase Pairs ........................................................................... 5-21 5.2.3 Step 3: Calculate the Critical Volume .............................................................................. 5-24 5.2.4 Step 4: Estimate the Cycle Length .................................................................................... 5-25 5.3 ROLE OF SOFTWARE IN SIGNAL TIMING ..................................................................... 5-26 5.3.1 Types of Software Models ................................................................................................... 5-26 5.3.2 Software Considerations ...................................................................................................... 5-28 5.3.3 Software Inputs and Outputs ............................................................................................. 5-30 5.4 REFERENCES ......................................................................................................................... 5-31 Signal Timing Manual, Second Edion

Chapter 5. Introducon to Timing Plans LIST OF EXHIBITS Exhibit 5-1 Typical Movement and Phase Numbering (with Protected Left Turns) ................................................................................................................................ 5-2 Exhibit 5-2 Typical Movement and Phase Numbering (with Permitted Left Turns) ................................................................................................................................ 5-3 Exhibit 5-3 Basic Ring-and-Barrier Diagram ............................................................................ 5-4 Exhibit 5-4 Left-Turn Phasing Options ........................................................................................ 5-5 Exhibit 5-5 Ring-and-Barrier Diagram Showing Permitted Left-Turn Phasing ......... 5-6 Exhibit 5-6 Ring-and-Barrier Diagram Showing Protected Left-Turn Phasing .......... 5-7 Exhibit 5-7 Ring-and-Barrier Diagram Showing Protected-Permitted Left- Turn Phasing ................................................................................................................... 5-8 Exhibit 5-8 Ring-and-Barrier Diagram Showing Split Phasing .......................................... 5-9 Exhibit 5-9 Example Prohibited Left Turns by Time of Day ............................................ 5-10 Exhibit 5-10 Ring-and-Barrier Diagram Showing Protected Lead-Lag Left- Turn Phasing ................................................................................................................ 5-11 Exhibit 5-11 Example Overlap and Parent Phase ................................................................... 5-12 Exhibit 5-12 Typical Right-Turn Overlap Phase Lettering .................................................. 5-13 Exhibit 5-13 Typical Right-Turn Overlap Settings ................................................................. 5-13 Exhibit 5-14 Ring-and-Barrier Diagram Showing Overlaps (with a Pedestrian ModiŠier Function) .................................................................................................... 5-14 Exhibit 5-15 Example of Trailing Overlap Phase Lettering ................................................ 5-15 Exhibit 5-16 Ring-and-Barrier Diagram Showing Trailing Overlaps .............................. 5-16 Exhibit 5-17 Basic Detector Assignment .................................................................................... 5-17 Exhibit 5-18 Basic Vehicle Detector Numbering ..................................................................... 5-17 Exhibit 5-19 Typical Load Switch Assignment ......................................................................... 5-18 Exhibit 5-20 Example #1: Critical Movement Analysis Phases and Volumes ............. 5-19 Exhibit 5-21 Example #2: Critical Movement Analysis Phases and Volumes ............. 5-20 Exhibit 5-22 Example #1: ConŠlicting Phases ........................................................................... 5-21 Exhibit 5-23 Example #1: Volumes by Phase ........................................................................... 5-22 Exhibit 5-24 Example #1: Major and Minor Street Critical Volumes .............................. 5-22 Exhibit 5-25 Example #2: ConŠlicting Phases ........................................................................... 5-23 Exhibit 5-26 Example #2: Volumes by Phase ........................................................................... 5-23 Exhibit 5-27 Example #2: Major and Minor Street Critical Volumes .............................. 5-24 Exhibit 5-28 Example #1: Critical Volume ................................................................................. 5-25 Exhibit 5-29 Example #2: Critical Volume ................................................................................. 5-25 Exhibit 5-30 Estimated Cycle Lengths Based on Critical Volume (Eight-Phase Intersection) ................................................................................................................ 5-26 Exhibit 5-31 References for Typical Software Inputs and Outputs ................................. 5-30 Signal Timing Manual, Second Edion

Chapter 5. Introducon to Timing Plans 5-1 CHAPTER 5. INTRODUCTION TO TIMING PLANS Chapters 5, 6, and 7 make up a three-part series about developing signal timing plans. Chapter 5 describes basic signal timing concepts that a practitioner should understand before deining signal timing values. Chapter 6 provides detailed information about signal timing parameters required at every signalized intersection, and Chapter 7 describes the timing parameters that must be deined when signalized intersections are coordinated. Using the information from Chapters 5, 6, and 7, a practitioner should be able to develop a timing plan that meets a deined timing strategy and local operational objectives (discussed in Chapter 3). 5.1 BASIC SIGNAL TIMING CONCEPTS There are many signal timing parameters that must be deined for every user group at an intersection for every time period throughout the day. In order to keep the parameters organized, practitioners have developed conventions for how movements are referenced, how phases are numbered, how overlaps work, and how those movements, phases, and overlaps correspond to detectors, signal cabinet equipment, and displays. 5.1.1 Movement and Phase Numbering Movements describe user actions at an intersection. At a signalized intersection (with four approaches), it is possible to have twelve one-way vehicular movements and four two-way pedestrian movements. Each of these movements can be assigned a number for reference. The Highway Capacity Manual (HCM) (1) assigns movement numbers as shown in Exhibit 5-1 (illustrated by the gray squares). The HCM gives each right-turn movement its own number (separate from the through movement) by adding 10 to the adjacent through movement number. It is important for a practitioner to understand the difference between lane assignments and movements. Note that a single movement can be accommodated by multiple lanes (e.g., through movement in two lanes), or multiple movements can be accommodated by a single lane (e.g., through/right-turn lane). A trafic signal phase is a timing process, within the signal controller, that facilitates serving one or more movements at the same time (for one or more modes of users). A practitioner must assign phase numbers to the movements at a signalized intersection in order to begin selecting signal timing values. A typical four-legged intersection with protected left-turn movements (protected movements have the right-of-way over other movements) will generally follow the phase numbering shown in Exhibit 5-1 (illustrated by the blue boxes). This standard National Electric Manufacturers Association (NEMA) phase numbering system combines the through movements with the right-turn movements, which are typically permitted (meaning they can be made after yielding to conlicting bicycle and pedestrian movements). Occasionally, the right- turn movement may be a protected movement and timed using an overlap, which is discussed in Section 5.1.4. To further explain the relationship between movements and phases, Exhibit 5-2 illustrates the typical movement and phase numbering used at an intersection with permitted left-turn movements (i.e., no protected left-turn phases). In this scenario, all Developing signal ming plans involves selecng ming values that ulmately determine how an intersecon (or more commonly a system of intersecons) operates. When developing a signal ming plan, movements should be the first focus. Signal phasing can be assigned to carry out the desired operaons. Signal Timing Manual, Second Edi€on

5-2 Chapter 5. Introduc on to Timing Plans of the movements on an approach are assigned to one phase. Permitted movements are shown as dashed arrows in Exhibits 5-1 and 5-2. Note that because of the concurrent through vehicle movements and parallel pedestrian phases, right-turn movements in both exhibits are shown as permitted because they must yield to through bicycles and pedestrians. Under a typical phase numbering scheme, there are several conventions that a practitioner should attempt to follow: • Even phases are typically associated with through movements. o Phases 2 and 6 generally represent the major street through movements. o Phases 4 and 8 generally represent the minor street through movements. • Odd phases are typically associated with left-turn movements. o Phases 1 and 5 generally represent the major street left-turn movements. Exhibit 5-1 Typical Movement and Phase Numbering (with Protected Le Turns) Signal Timing Manual, Second Edion

Chapter 5. Introducon to Timing Plans 5-3 o Phases 3 and 7 generally represent the minor street left-turn movements. • Pedestrian phases are typically set up to run concurrently with the even- numbered vehicular phases. They are generally assigned the same phase number as the adjacent, parallel vehicular phases. o Pedestrian Phases 2 and 6 generally represent the major street pedestrian movements. o Pedestrian Phases 4 and 8 generally represent the minor street pedestrian movements. To avoid confusion, it is common practice to maintain a consistent phase numbering scheme within a speciic jurisdiction. For example, Phase 2 could always be deined as the major street through movement in the northbound (or eastbound) direction, or alternatively, as the coordinated phase (regardless of direction). This manual will always follow the convention that Phases 2 and 6 are the major street phases and that they are the coordinated phases when under coordinated operations. Phase Exhibit 5-2 Typical Movement and Numbering (with Permied Le Turns) Signal Timing Manual, Second Edion

5-4 Chapter 5. Introduc on to Timing Plans 5.1.2 Ring-and-Barrier Concept Rings and barriers fundamentally deine how a controller organizes phases, so that compatible phases can time together and conlicts do not occur. • Ring: A ring shows a sequence of conlicting phases. Dual (or two) ring operations allow compatible phases to operate concurrently with (i.e., at the same time as) phases in another ring. • Barrier: A barrier is the point at which the phases in both rings must end simultaneously. Barriers typically separate major and minor street phases. The example ring-and-barrier diagram in Exhibit 5-3 has two rings and two barriers, which organize eight phases. Note that the left-turn phases are protected and leading (i.e., preceding the through movements), which is the most common phasing for an intersection with four approaches and protected left-turn phasing. Other left-turn phasing alternatives are discussed in the following section. Phases within two barriers are known as a compatible phases (as shown in Exhibit 5-3). Between the barriers, several rules apply: • Any phase in Ring 1 can time with any phase in Ring 2. For example (in Exhibit 5-3), Phases 1, 2, 5, and 6 are compatible phases, so Phases 1 or 2 can time with Phases 5 or 6. Similarly, Phases 3, 4, 7, and 8 are compatible phases, so Phases 3 or 4 can time with Phases 7 or 8. • Any phase in a ring can be skipped and/or give unused time to a following phase in that ring. For example (in Exhibit 5-3), Phase 1 can give time to Phase 2, and Phase 5 can give time to Phase 6. Alternatively, Phases 3 and 7 can be skipped if there is no demand. • Subject to additional rules described in Chapter 6, it is possible for only one ring to have a phase timing (i.e., in the other ring, all phases are resting in red). Rings and barriers allow phases to me independently and flexibly, yet within a structure. Exhibit 5-3 Basic Ring- and-Barrier Diagram Signal Timing Manual, Second Edion

Chapter 5. Introducon to Timing Plans 5-5 5.1.3 Le-Turn Phasing There are ive options for left-turn phasing at an intersection (summarized in Exhibit 5-4): permitted, protected, protected-permitted, split phasing, and prohibited. In general, the type of phasing used for one left-turn movement is also used for its opposing left-turn movement. For example, if one left-turn movement is permitted, the opposing left turn is also generally permitted. However, this is not a requirement; the left-turn phasing should be movement-speciic and chosen based on a variety of operational and safety factors (discussed in detail in Chapter 4). A practitioner should consult local jurisdiction guidelines when determining which type of left-turn phasing to use at an intersection. Additional information about each left-turn phasing option is provided in the following sections, followed by information on left-turn phase sequence options. Le-Turn Phasing Op on Descrip on Advantages Challenges Permied Le- Turn Phase Served with the adjacent through movement, requiring le-turning vehicles to yield to conflicng vehicle and pedestrian movements □ Reduced intersecon delay □ Efficient green allocaon □ Requires users to choose acceptable gaps in traffic □ Yellow trap can occur if opposing movement is a lagging le turn Protected Le- Turn Phase Le-turning vehicles are given the right-of-way without any conflicng movements □ Reduced delay for le- turning vehicles □ Users always receive exclusive right-of-way; gaps in traffic do not need to be idenfied □ Increased intersecon delay Protected- Permied Le- Turn Phase Combinaon of permied and protected le-turn phasing; users receive a protected interval, but can also make permied movements as the conflicng through phase receives a green indicaon □ Compromise between safety of protected le- turn phase and efficiency of permied le-turn phase □ No significant increase in delay for other movements □ Fewer opons for maximizing progression of through vehicles during coordinaon (unless flashing yellow arrow displays are used) □ Yellow trap can occur if opposing movement is a lagging le turn Split Phase Assignment of right-of-way to all movements of a parcular approach, followed by all of the movements of the opposing approach □ Accommodates use of shared lanes (e.g., le/through lane) □ Necessary when opposing le-turn paths overlap because of intersecon geometry □ Avoids conflict between opposing le-turning vehicles □ Increased coordinated cycle length, parcularly if both split phases have concurrent pedestrian phases □ Less efficient than other types of le-turn phasing Prohibited Le- Turn Phase Implemented to maintain mobility at an intersecon through use of a “no le turn” sign (parcularly during mes of day when gaps are unavailable) □ Reduced conflicts at the intersecon □ Users must find alternave routes Exhibit 5-4 Le-Turn Phasing Opons Signal Timing Manual, Second Edion

5-6 Chapter 5. Introduc on to Timing Plans 5.1.3.1 Permied Le-Turn Phasing Permitted left-turn phasing is depicted in the ring-and-barrier diagram in Exhibit 5-5 and has the following characteristics: • Right-of-Way: Permitted phasing requires a user to yield to conlicting vehicular and pedestrian trafic before completing a left turn. • Displays: Both the left-turn and opposing through movements are presented with a circular green indication (i.e., green arrow is never provided). • Intersection Conditions: Permitted operations are primarily used when trafic is light to moderate and sight distance is adequate. • Advantages: This display option provides the most eficient green time allocation, but the eficiency is dependent on the availability of gaps in the conlicting trafic. • Challenges: This mode can have an adverse effect on safety in some situations, such as when the left-turning vehicle’s view of conlicting trafic is restricted or when adequate gaps in trafic are not present. The yellow trap (see Chapter 4) can occur if the opposite direction has a lagging left-turn movement. 5.1.3.2 Protected Le-Turn Phasing Protected left-turn operations are shown in Exhibit 5-6 and have the following characteristics: • Right-of-Way: Protected left-turn phasing assigns the right-of-way to users turning left at the intersection. • Displays: It allows turns to be made only on a green arrow display. • Intersection Conditions: An exclusive left-turn lane is typically provided with this phasing. • Advantages: This operation provides for eficient left-turn service and is recognized as the safest type of left-turn operation. Exhibit 5-5 Ring-and- Barrier Diagram Showing Permied Le-Turn Phasing Signal Timing Manual, Second Edion

Chapter 5. Introducon to Timing Plans 5-7 • Challenges: The added left-turn phase increases the lost time within the cycle length and may increase delay for other movements. 5.1.3.3 Protected-Permied Le-Turn Phasing Protected-permitted left-turn operations are shown in Exhibit 5-7 and have the following characteristics: • Right-of-Way: Protected-permitted operations are a combination of the permitted and protected modes. Left-turning vehicles have the right-of-way during the protected left-turn phase and can also complete the left turn "permissively" when the adjacent through movement receives its circular green indication. • Displays: This type of operation requires the use of either a lashing yellow arrow (FYA) display or a ive-section “doghouse” display (see Chapter 4 for details). • Advantages: This mode provides for eficient left-turn service, often without causing a signiicant increase in delay to other movements. This mode also tends to provide a relatively safe left-turn operation as long as adequate sight distance is available. • Challenges: Protected-permitted left-turn phasing should be applied with caution when a phasing sequence other than lead-lead is used (see Section 5.1.3.6 for information on phase sequence). For protected-permitted phasing, the lead-lead sequence prevents the yellow trap associated with lagging left- turn phasing and ive-section heads (explained in Chapter 4). However, under light trafic conditions and in the absence of minor street trafic, left-turn phases may be re-serviced without crossing the barrier first, resulting in de facto lagging left turns and a potential yellow trap. Most modern trafic signal controllers have a feature that provides left-turn backup protection. It typically works by omitting the protected left-turn phase when the adjacent through movement is green, ensuring that the left-turn phase will always be preceded by Exhibit 5-6 Ring-and- Barrier Diagram Showing Protected Le-Turn Phasing Signal Timing Manual, Second Edion

5-8 Chapter 5. Introduc on to Timing Plans a barrier. The name of this feature varies by different traf ic signal controllers, but some common names include Five-Section Head Restrictions, Five-Section Logic, Trap Protected Phase, Backup Prevent Phases, and Anti-Backup. Protected-permitted operations can also be challenging when used along a coordinated corridor. Additional opportunities for left-turn movements mean fewer opportunities for through vehicles to progress along a corridor. Chapter 7 provides more information on coordination considerations. 5.1.3.4 Split Phasing Split phasing is depicted in the ring-and-barrier diagram in Exhibit 5-8 and has the following characteristics: • Right-of-Way: Split phasing assigns right-of-way to all movements on a particular approach, followed by all of the movements on the opposing approach. • Displays: Split phasing uses the same type of displays as protected operations, but requires additional programming in the controller. • Intersection Conditions: Split phasing may be necessary when the following conditions are present (2): o There is a need to accommodate one or more left-turn lanes on opposing approaches, but suf icient width is not available to ensure adequate separation between vehicle paths in the middle of the intersection. This condition may also be caused by a large intersection skew angle. o The larger left-turn lane volume is equal to its opposing through lane volume during most hours of the day. (“Lane volume” represents the movement volume divided by the number of lanes serving it.) o The width of the road is constrained such that an approach lane is shared by the left-turn and through movements, yet the left-turn volume is suf icient to justify a left-turn phase. Exhibit 5-7 Ring-and- Barrier Diagram Showing Protected- Permied Le-Turn Phasing Signal Timing Manual, Second Edion

Chapter 5. Introducon to Timing Plans 5-9 o One of the two approaches has a heavy volume, the other approach has minimal volume, and actuated control is used. In this situation, the phase associated with the low-volume approach would rarely be called, and the intersection would mostly function as a T-intersection. o Crash history indicates an unusually large number of sideswipe or head- on crashes in the middle of the intersection involving left-turning vehicles. • Advantages: Split phasing prevents conlicts between opposing left-turning vehicles. • Challenges: This phasing is generally less eficient than other types of left-turn phasing. It typically increases the cycle length or, if the cycle length is ixed, reduces the time available to the intersecting road (2). 5.1.3.5 Prohibited Le-Turn Phasing Prohibiting left-turn movements is a phasing option with the following characteristics: • Right-of-Way: Left turns can be prohibited during some or all times of day. • Displays: If left turns are prohibited during all times of day, no signal displays are needed, but a “No Left Turn” sign is required. If left turns are prohibited only during certain times of day (when gaps in trafic are unavailable and permitted phasing may be unsafe), displays should be chosen based on the appropriate phasing, and a supplemental sign should be added. Exhibit 5-9 shows an example of such a sign in Toronto, Ontario, where left turns are prohibited during the morning and evening periods. • Advantages: Prohibiting left turns can help to maintain mobility at an intersection. • Disadvantages: Vehicles wanting to make a left turn must ind alternative routes. Exhibit 5-8 Ring-and- Barrier Diagram Showing Split Phasing Signal Timing Manual, Second Edion

5-10 Chapter 5. Introduc on to Timing Plans 5.1.3.6 Le-Turn Phase Sequence Regardless of the type of left-turn phasing that is applied, it may be advantageous under certain conditions to change the sequence in which left-turn phases are served (relative to the through phases). There are three sequence options available: lead-lead, lag-lag, and lead-lag. The most common left-turn phase sequence is the lead-lead sequence, which starts opposing left-turn phases prior to the through phases (see Exhibit 5-6 for an example). The advantages of this sequence option include the following: • Users react quickly to the leading green arrow indication. • It minimizes con€licts between left-turn and through movements on the same approach, especially when the left-turn volume exceeds the available storage bay (or no left-turn lane is provided). • It gives unused time to the through movements. The lag-lag sequence, on the other hand, serves opposing left-turn phases after the through phases. This sequence is most often used in coordinated systems with closely spaced signals (e.g., diamond interchange). Lagging the left-turn phases can have operational bene€its when there is an unopposed, protected-permitted left-turn phase (e.g., at a T-intersection or at the intersection of a two-way street and a one-way street). However, the disadvantages of lagging the left-turn phases include the following: • Users tend not to react as quickly to the lagging green arrow indication. • If a left-turn lane does not exist (or is relatively short), queued left-turn vehicles may block the inside through lane during the initial through movement phase. • When lag-lag phasing is used at an intersection with four approaches, where opposing phases are protected-permitted, a yellow trap could exist. (Note that the yellow trap can be alleviated by using a protected-only left-turn phase or a FYA display.) Exhibit 5-9 Example Prohibited Le Turns by Time of Day Signal Timing Manual, Second Edion

Chapter 5. Introducon to Timing Plans 5-11 Opposing left turns can also run in a lead-lag sequence, where they begin and end at different times relative to the through phases (as shown by Phases 1 and 5 in Exhibit 5-10). This sequence is generally used to accommodate through movement progression in a coordinated signal system (discussed in Chapter 7). Like the lag-lag sequence, a yellow trap may exist if protected-permitted phasing is used and should be considered when programming the intersection. Lead-lag phasing can have operational bene‚its for the following conditions: • Where there is inadequate space in the intersection to safely accommodate simultaneous service of opposing left-turn movements. (An appropriate controller con‚iguration should be used to ensure that the left-turn phases never time concurrently.) • Intersections where the leading left-turn movement is not provided with an exclusive lane (or the available left-turn storage bay is relatively small). 5.1.4 Overlaps Overlaps provide a way to operate movement(s) with one or more phases. While overlaps are often incorrectly associated with hard-wiring multiple movements to the same phase in the signal cabinet (e.g., right-turn arrow wired to compatible left-turn phase), true overlaps require their own load switch and accompanying signal timing. Overlaps can vary widely in their capabilities and how they time speci‚ic intervals. When using overlaps, bench testing is always the best approach to understanding how the overlap actually times its output. Overlaps are controlled through parent phases (which typically add phases together) and modi‚ier phases (which typically exclude operation during modi‚ier phases) and allow non-con‚licting movements to receive a green according to the overlap rules (as demonstrated by the example in Exhibit 5-11). While the right-turn overlap movement in this example could be wired directly to the left-turn parent phase (and, therefore, incorrectly be called an overlap because it achieves the same result), eliminating the use of an overlap load switch decreases ‚lexibility in the signal timing. Exhibit 5-10 Ring-and- Barrier Diagram Showing Protected Lead-Lag Le-Turn Phasing An overlap is a separate output that can use special logic to improve opera ons. Signal Timing Manual, Second Edion

5-12 Chapter 5. Introduc on to Timing Plans Overlaps can be used to combine phases for any non-con licting movements, but they are most often used for right-turn movements where exclusive right-turn lanes exist. For right-turn overlaps, the parent phase is typically the compatible left-turn phase. However, it is also possible to operate the right-turn overlaps (subject to the pedestrian con licts discussed below) with both the adjacent through phase and the compatible left-turn phase. Exhibit 5-12 illustrates a common phase lettering scheme for right-turn overlaps. (Note that overlaps are also sometimes numbered.) Some traf ic signal controllers have a feature that allows a right-turn overlap to be omitted when the con licting pedestrian phase is active (through the use of a modi ier pedestrian phase). This feature is needed if a pedestrian phase is associated with the through vehicular movement. If the right-turn arrow is allowed to turn green at the same time as the through display (and a pedestrian call has been placed), right-turning vehicles will be in con lict with pedestrians receiving a walk indication. An overlap modi ier feature will allow the right-turn overlap to have both the compatible left-turn and adjacent through movements as parent phases. Assigning the con licting pedestrian phase as the modi ier phase (also called Pedestrian Protect, Not Ped Overlap, Con licting Ped, Negative Pedestrian Phase, or Omit on Green) excludes the right-turn overlap only when there is a pedestrian call on the adjacent through movement. Without the pedestrian modi ier, the right-turn overlap must run as a permitted movement with the adjacent through movement to avoid pedestrian con licts (which is less ef icient). Exhibit 5-13 provides a summary of the typical right-turn overlap settings at a standard eight-phase intersection, assuming overlaps are provided for all four right-turn movements. The parent phase designations assume the phase numbering scheme illustrated in Exhibit 5-12. Exhibit 5-11 Example Overlap and Parent Phase Signal Timing Manual, Second Edion

Chapter 5. Introducon to Timing Plans 5-13 1 Movement Number Overlap Leer1 Parent Phase Pedestrian Modifier Phase for Right-Turn Overlap Omit (If Available) 12 A 2* & 3 2P 14 B 4* & 5 4P 16 C 6* & 7 6P 18 D 8* & 1 8P Agencies may have different overlap assignments based on their preference. * These phases should not be included as parent phases if a controller feature to omit right-turn overlap with active conflicting pedestrian phases is not available. Exhibit 5-14 is a ring-and-barrier diagram that shows right-turn overlaps on the major street (during Phases 2 + 3 and Phases 6 + 7). In this example, the right-turn overlaps use a pedestrian modi ier function so that the right-turn movements do not con lict with pedestrians utilizing Pedestrian Phases 2 and 6. Assuming that the major street right-turn movements have protected-permitted displays, Overlaps A and C (highlighted in blue) can operate in two different ways during Phases 2 and 6. When a Exhibit 5-12 Typical Right-Turn Overlap Phase Le„ering Exhibit 5-13 Typical Right-Turn Overlap Se†ngs Signal Timing Manual, Second Edi„on

5-14 Chapter 5. Introduc on to Timing Plans pedestrian phase is called, the adjacent right-turn movement will be permitted (i.e., green ball), and, when there are no pedestrians, the right-turn movement will be protected (i.e., green arrow). Note that if a protected-only display is used with a pedestrian modiier function, the right-turn vehicular movement will be omitted when the conlicting pedestrian phase is called, and a right-turn red arrow will be displayed. A trailing overlap is an overlap application that is commonly used at closely spaced intersections (e.g., two closely spaced T-intersections). A trailing overlap continues timing after the parent phase ends. Exhibit 5-15 shows an example intersection where trailing overlaps may be applied. The movements approaching the two-intersection system are assigned phases in a typical fashion (similar to the example in Exhibit 5-1). The movements between the intersections are what differentiates this type of operation. In this case, the southbound movement is assigned to Overlap A (which has Phase 2 as its parent phase), and the northbound movement is assigned to Overlap B (which has Phase 6 as its parent phase). Trailing overlaps are different from standard overlaps because they time with the parent phase plus a speciied amount of time after the parent phase ends. Exhibit 5-16 shows an example ring-and-barrier diagram associated with the intersection depicted in Exhibit 5-15. Note that the ring-and-barrier diagram illustrates movements for both intersections, using a dashed line in each ring to separate the movements at the northern intersection from those at the southern intersection. In this example, the trailing overlaps (Overlaps A and B) allow for better progression of vehicles between the intersections, reducing the likelihood that a vehicle will get “stuck” between the intersections. The trailing overlaps essentially allow for a set period of time when the space between the intersections is cleared. Exhibit 5-14 Ring-and- Barrier Diagram Showing Overlaps (with a Pedestrian Modifier Func on) Signal Timing Manual, Second Edion

5-14 Chapter 5. Introduc on to Timing Plans pedestrian phase is called, the adjacent right-turn movement will be permitted (i.e., green ball), and, when there are no pedestrians, the right-turn movement will be protected (i.e., green arrow). Note that if a protected-only display is used with a pedestrian modiier function, the right-turn vehicular movement will be omitted when the conlicting pedestrian phase is called, and a right-turn red arrow will be displayed. A trailing overlap is an overlap application that is commonly used at closely spaced intersections (e.g., two closely spaced T-intersections). A trailing overlap continues timing after the parent phase ends. Exhibit 5-15 shows an example intersection where trailing overlaps may be applied. The movements approaching the two-intersection system are assigned phases in a typical fashion (similar to the example in Exhibit 5-1). The movements between the intersections are what differentiates this type of operation. In this case, the southbound movement is assigned to Overlap A (which has Phase 2 as its parent phase), and the northbound movement is assigned to Overlap B (which has Phase 6 as its parent phase). Trailing overlaps are different from standard overlaps because they time with the parent phase plus a speciied amount of time after the parent phase ends. Exhibit 5-16 shows an example ring-and-barrier diagram associated with the intersection depicted in Exhibit 5-15. Note that the ring-and-barrier diagram illustrates movements for both intersections, using a dashed line in each ring to separate the movements at the northern intersection from those at the southern intersection. In this example, the trailing overlaps (Overlaps A and B) allow for better progression of vehicles between the intersections, reducing the likelihood that a vehicle will get “stuck” between the intersections. The trailing overlaps essentially allow for a set period of time when the space between the intersections is cleared. Exhibit 5-14 Ring-and- Barrier Diagram Showing Overlaps (with a Pedestrian Modifier Func on) Signal Timing Manual, Second Edion Chapter 5. Introducon to Timing Plans 5-15 Exhibit 5-15 Example of Trailing Overlap Phase Leering Signal Timing Manual, Second Edion

5-16 Chapter 5. Introduc on to Timing Plans 5.1.5 Detector Assignments Detector assignments deine how the controller will react when detector inputs are received from the ield. Basic functions of a detector include calling and/or extending a phase (discussed in detail in Chapter 6). At a fully-actuated intersection, at least one detector is needed to call and extend each phase. The “call” function becomes active only when the phase is not in its green interval, while the “extend” function extends the phase through the use of a timer only during the green interval. Just as phases and movements are assigned numbers, detectors are also assigned numbers. It is essential to know which number corresponds to each detector so that the detectors can be assigned to the appropriate phase (or phases) in the controller. Exhibit 5-17 summarizes a simple detector assignment at an eight-phase intersection with fully- actuated operations and one detector per lane (as shown in Exhibit 5-18). (Note that most agencies develop a standard that accounts for multiple detectors per phase.) Each detection zone (even those associated with the same phase) will have unique numbers for identiication in the signal cabinet. Regardless of the detection numbering scheme that is applied, an agency should use a consistent approach for assigning detector numbers in order to facilitate maintenance and minimize errors. Exhibit 5-16 Ring-and- Barrier Diagram Showing Trailing Overlaps Signal Timing Manual, Second Edion

Chapter 5. Introducon to Timing Plans 5-17 Phase Detector Number1 Func on Vehicle Phase 1 1 Call & Extend Vehicle Phase 2 2 Call & Extend Vehicle Phase 2 12 Call Vehicle Phase 3 3 Call & Extend Vehicle Phase 4 4 Call & Extend Vehicle Phase 4 14 Call Vehicle Phase 5 5 Call & Extend Vehicle Phase 6 6 Call & Extend Vehicle Phase 6 16 Call Vehicle Phase 7 7 Call & Extend Vehicle Phase 8 8 Call & Extend Vehicle Phase 8 18 Call 1 Detector number will vary depending on field wiring. Most trafic signal controllers do not require additional settings for pedestrian detector assignment. Pedestrian detectors are typically wired such that they are automatically associated with pedestrian phases through a standard input/output Exhibit 5-17 Basic Detector Assignment Exhibit 5-18 Basic Vehicle Detector Numbering Signal Timing Manual, Second Edi on

5-18 Chapter 5. Introduc on to Timing Plans Phase / Overlap Load Switch Number Vehicle Phase 1 1 Vehicle Phase 2 2 Vehicle Phase 3 3 Vehicle Phase 4 4 Vehicle Phase 5 5 Vehicle Phase 6 6 Vehicle Phase 7 7 Vehicle Phase 8 8 Overlap A 9 Overlap B 10 Overlap C 11 Overlap D 12 Pedestrian Phase 2 + FYA1* 13 Pedestrian Phase 4 + FYA3* 14 Pedestrian Phase 6 + FYA5* 15 Pedestrian Phase 8 + FYA7* 16 * FYA displays often use the pedestrian phase yellow load switch. Signal monitors are responsible for monitoring conflic ng phases and conflic ng indica ons in a single display. (More informa on is available in Chapter 4.) Exhibit 5-19 Typical Load Switch Assignment coniguration (speciic to the type of cabinet being used). However, if there is an exclusive pedestrian phase, the standard settings will need to be adjusted for the appropriate phase, consistent with the capabilities of the controller. Changes in phase sequence that differ from the standard phasing scheme of Phases 2, 4, 6, and 8 being the parallel through movements to companion pedestrian phases may also require adjustment of the pedestrian detector inputs. 5.1.6 Load Switch Assignments In addition to assigning detectors to the phases at an intersection, a practitioner must also associate the displays with each phase. The association between phases/overlaps and vehicular/pedestrian movements is deined by the programmed signal phasing in the controller irmware, the load switch assignment in the trafic signal controller cabinet, and the wiring between each load switch and the signal displays. Most modern trafic signal controller cabinets have default load switch assignments, supported by typical controller firmware, for standard eight-phase intersections. Therefore, additional settings or changes are generally not needed for standard applications. Intersections with eight vehicular phases, four pedestrian phases, and four overlap movements might have the default load switch assignment shown in Exhibit 5-19. Other default settings would be determined by the controller irmware version. Load switches each have three outputs that are generally associated with red, yellow, and green indications. Pedestrian displays use the same type of load switch as vehicle displays, but they only use the “green” output for walk and the “red” output for don’t walk. Flashing don’t walk is the “red” output lashed on and off every half second. For indications with more than three outputs, such as an FYA display, unused load switch outputs can be reassigned. For an FYA display, the lashing yellow indication is typically run off an unused load switch output (e.g., pedestrian phase “yellow”) or an unused load switch (e.g., an entirely unused overlap or phase). The use of the pedestrian “yellow” output is the most eficient use of load switches, but it is not possible with some controllers, and may not be compatible with some signal monitors. The practitioner will have to verify compatibility before reassigning load switches. Signal Timing Manual, Second Edi­on

Chapter 5. Introducon to Timing Plans 5-19 5.2 CRITICAL MOVEMENT ANALYSIS An important principle behind effective signal timing plans is the relationship between signal timing, phasing, and the capacity of an intersection. A variety of analysis procedures, ranging from simple to complex, can be used to evaluate signalized intersection performance. Critical movement analysis is a simpliied technique that has broad applications for estimating phasing needs and signal timing parameters. This method allows a practitioner to identify the critical phase pairs at an intersection, calculate the critical volume, and approximate the required cycle length. The method is generally simple enough to be conducted by hand (making it convenient for use in the ield), although some of the more complicated reinements are aided considerably by the use of a simple spreadsheet. In many cases (such as signals at new streets), critical movement analysis is the appropriate level of analysis given the lack of accuracy in future volume forecasts. It can also provide a irst-cut reasonableness check of software results, and, therefore, is an important procedure to understand as a signal timing practitioner. The following sections demonstrate the critical movement analysis procedure using two simple examples. While both examples assume the same lane coniguration and vehicular volumes at the intersection, they assume different phase sequences. The irst example demonstrates the critical movement analysis procedure using a typical eight- phase intersection with protected left turns on all approaches (Example #1 shown in Exhibit 5-20), while the second example uses an intersection with split phasing on the minor street (Example #2 shown in Exhibit 5-21). Although more complex methodologies exist, cri cal movement analysis may be the most appropriate for certain situa ons. Exhibit 5-20 Example #1: Crical Movement Analysis Phases and Volumes Signal Timing Manual, Second Edion

5-20 Chapter 5. Introduc on to Timing Plans 5.2.1 Step 1: Record Demand Volumes The irst step in the critical movement analysis procedure is determining peak period volumes in each lane at the intersection (as shown in Exhibit 5-20 and Exhibit 5-21). It should be noted that the results of an analysis are highly dependent on the accuracy and also the variability in trafic volumes. Care should be taken in interpreting the results, and the interpretation should be based on an understanding of the fundamental quality of the data. There are several adjustments that should be made to the volumes depending on site-speciic conditions: • Peak-Hour Adjustment. If hourly trafic volumes are being used, they are usually adjusted to relect the peak 15-minute period. • Heavy-Vehicle Adjustment. The practitioner should always evaluate the proportion of heavy vehicles and make adjustments to account for larger vehicles if appropriate. Modest heavy-vehicle volumes are assumed in this example, so no adjustments have been made. If signiicant numbers have been counted, trucks and buses can be considered as two vehicles each in the analysis. • Lane Imbalance Adjustment. Volumes should be adjusted to account for site- speciic conditions if signiicant lane imbalances exist due to downstream destinations. In this example, volumes are evenly distributed among the lanes. Exhibit 5-21 Example #2: Cri cal Movement Analysis Phases and Volumes Signal Timing Manual, Second Edion

Chapter 5. Introducon to Timing Plans 5-21 5.2.2 Step 2: Determine Crical Phase Pairs The cycle length needed to accommodate all of the vehicles at an intersection can be estimated by identifying the movements that require the most time (the critical movements). In the second step of the critical movement analysis procedure, the vehicular volumes associated with conlicting phases (generally a left-turn phase and opposing through movement phase) are used to determine the critical phase pairs. 5.2.2.1 Example #1: Protected Le -Turn Phasing The volume sums for all sets of conlicting phases are compared between the barriers (in Example #1, Phases 1 + 2, Phases 3 + 4, Phases 5 + 6, and Phases 7 + 8), as depicted in the ring-and-barrier diagram in Exhibit 5-22. This comparison allows a practitioner to identify the critical movements (and associated critical volumes) for the major street and minor street. In Example #1, the amount of time given to the major street will be dictated by whichever conlicting set of phases (Phases 1 + 2 or Phases 5 + 6) has the most volume. Similarly, the amount of time given to the minor street will be dictated by whichever conlicting set of phases (Phases 3 + 4 or Phases 7 + 8) has the most volume. This will ensure that the cycle length is based on the movements with the most volume (that thus require the most time). Exhibit 5-23 shows the volumes from Exhibit 5-20 assigned to each phase in the ring-and-barrier diagram. Note that all of the through movements are associated with two lanes. The highest volume in any lane associated with the same phase should be used to identify the critical movements. Typically, through lanes will experience fairly uniform volumes, but there are exceptions to this, such as at locations near freeway interchanges. As an example, note that there are two through lanes assigned to Phase 2 in Exhibit 5-20. If the igure showed that there were 385 through vehicles in the inside lane and 585 through vehicles in the outside lane, the Phase 2 volume used for the conlicting phase volume calculation would be 585 vehicles. Exhibit 5-22 Example #1: Conflicng Phases Signal Timing Manual, Second Edion

5-22 Chapter 5. Introduc on to Timing Plans In Example #1, Phases 1 and 2 have more con licting volume than Phases 5 and 6 (as depicted in Exhibit 5-24). In other words, the volume proceeding through the intersection during Phases 5 and 6 will be able to clear the intersection before the volume during Phases 1 and 2, so Phases 1 and 2 are the major street critical movements. Phases 3 and 4 have more con licting volume than Phases 7 and 8, so they are the minor street critical movements. 5.2.2.2 Example #2: Split Phasing on the Minor Street Note that different phase sequences may change which phases are considered in the critical movement analysis. In Example #2, split phasing at the intersection allows all movements from one minor street approach to proceed through the intersection at the same time (Phase 3), followed by all the movements from the other minor street Exhibit 5-23 Example #1: Volumes by Phase Exhibit 5-24 Example #1: Major and Minor Street Cri cal Volumes Signal Timing Manual, Second Edion

Chapter 5. Introducon to Timing Plans 5-23 approach (Phase 4). Exhibit 5-25 shows that the critical volume for the minor street will be determined by summing the greatest volume from each minor street approach. As a result of the split phasing, the minor street demand will likely require more time to serve than if the minor street were using permitted or protected left-turn phasing. In Example #2, the major street critical movements will be identi€ied using the same method explained in Example #1 (whichever con€licting set of phases has the most volume between Phases 1 + 2 or Phases 5 + 6). However, determining the critical movements on the minor street will involve a slightly different computation (as depicted in Exhibit 5-26). The minor street is using only two phases (Phases 3 and 4) because all of the vehicles from one approach will proceed through the intersection before all of the Exhibit 5-25 Example #2: Conflicng Phases Exhibit 5-26 Example #2: Volumes by Phase Signal Timing Manual, Second Edion

5-24 Chapter 5. Introduc on to Timing Plans vehicles from the other approach will proceed through the intersection. The two phases are in the same ring because they cannot run at the same time. (There are other ways to accomplish this, but this is the most common.) The amount of time required for the minor street will be dictated by the sum of the highest lane volumes on each minor street approach. As explained previously, the major street critical volume will be the same in Examples #1 and #2. The minor street critical volume in Example #2, however, is determined by adding the highest lane volume from each minor street approach (as depicted in Exhibit 5-27). The through volumes are higher than the left-turn volumes for both phases (Phases 3 and 4), so the through volumes will determine how much time is required for the minor street. During each minor street phase, left-turning vehicles should be able to clear the intersection before the through vehicles. Phases 3 and 4 run separately under split phasing, so the minor street critical volume is the sum of the highest lane volumes for Phases 3 and 4. 5.2.3 Step 3: Calculate the Cri cal Volume The third step in the critical movement analysis procedure is determining the total demand on the intersection. The critical volume on the major street and the critical volume on the minor street should be summed to determine the critical intersection volume (as shown in Exhibit 5-28 for Example #1 and Exhibit 5-29 for Example #2). The critical intersection volume is the highest number of vehicles that must be accommodated at the intersection based on the phase sequence and demand on each movement. Note that the critical volume for Example #2 is higher than the critical volume for Example #1, despite the same lane volumes from Exhibit 5-20 and Exhibit 5-21. More time will be required for the minor street under split phasing because vehicles from each minor street approach must proceed through the intersection separately. There is less ef‰iciency with this phase order. Exhibit 5-27 Example #2: Major and Minor Street Cri cal Volumes Signal Timing Manual, Second Edi on

Chapter 5. Introducon to Timing Plans 5-25 5.2.4 Step 4: Esmate the Cycle Length The critical volume at an intersection can be used to estimate the required cycle length. Exhibit 5-30 shows the number of vehicles that can be accomodated per cycle and per hour for an intersection with eight phases (based on cycle lengths between 60 seconds and 120 seconds). Effective green accounts for start-up lost time as well as vehicles moving during a portion of the yellow. Start-up lost time is the additional time consumed by the  irst few vehicles in a queue above and beyond the saturation headway, because of the need to react to the green indication and accelerate. Effective green is approximately equal to actual green in typical applications (i.e., no excessive start-up lost time or excessively long clearance intervals). The calculations assume an intersection capacity of 1,400 vehicles and 5 seconds of lost time per phase. In Example #1 (with eight phases and a critical volume of 1,135 vehicles), a cycle length of 110 seconds would accommodate the intersection demand based on a relatively conservative set of assumptions. (These conservative assumptions will generally provide an upper bound on cycle length.) The signal timing practitioner should use judgment when selecting signal timing values based on simple demand analysis, as the computed values may not be consistent with operational objectives. Intersection demand is just one factor that should affect the cycle length that is chosen. For example, the average segment length, street classi ication, left-turn phasing, pedestrian phasing, and objectives determined for the intersection can also in luence the optimum cycle length. If block lengths are long, a Exhibit 5-28 Example #1: Crical Volume Exhibit 5-29 Example #2: Crical Volume Signal Timing Manual, Second Edion

5-26 Chapter 5. Introduc on to Timing Plans longer cycle length may be appropriate to favor major street traf ic. If blocks are short or pedestrian volumes high, shorter cycle lengths may be more appropriate. Cycle Length (Seconds) Number of Cycles Per Hour Lost Time Per Cycle (Seconds)1 Effecve Green Time Per Cycle (Seconds) Number of Vehicles Per Cycle2 Maximum Number of Vehicles Per Hour2 60 60 20 40 16 933 70 51 20 50 19 1000 80 45 20 60 23 1050 90 40 20 70 27 1089 100 36 20 80 31 1120 110 33 20 90 35 1145 120 30 20 100 39 1167 1 This lost time assumes that the intersection is operating with eight phases (four in each ring) with 5 seconds of lost time per phase. The lost time will be less at an intersection with fewer phases. 2 The number of vehicles that can be accommodated under the various cycle lengths was calculated assuming a headway of 2.5 seconds per vehicle, which is generally conservative for urban/suburban environments. It should be noted that the longer the green time per cycle, often the larger the vehicle headways (i.e., lower vehicle density) later in the phase. This can result in less ef icient use of green time at the intersection, which is akin to lost time when aggregated. Longer cycle lengths are an effective way to move large numbers of vehicles at an intersection if the desired headways are maintained during the entire green time. Queue storage, upstream and downstream bottlenecks, and density of vehicles all play into effective movement of vehicles through a signalized intersection, and the needs of all users should be considered when selecting a cycle length for implementation. 5.3 ROLE OF SOFTWARE IN SIGNAL TIMING Computer-based tools are available to calculate and evaluate signal timing. It is important to recognize their capabilities and limitations, and it is recommended that practitioners develop a thorough understanding of the selected computer programs, their uses, and how they relate to ield conditions. Using the optimize feature of a software product that does not use an agency’s operational objectives will not produce the desired outcome. In other words, a sophisticated tool in the hands of an inexperienced analyst may not produce a satisfactory result. 5.3.1 Types of SoŠware Models In general, there are two types of models that can be developed using software tools: (1) deterministic or equation-based models and (2) microscopic simulation models. These two types of models assess different levels of detail related to traf ic operations, and should be applied to signal timing projects based on the traf ic conditions and complexity of the network. It should be noted that all models are only as good as the inputs, as well as their imbedded parameters. Saturation low rates, regardless of whether they are fixed or adjustable, are generally assumed to be constant, which is not necessarily the case as cycle length increases or access disrupts low. Exhibit 5-30 Es mated Cycle Lengths Based on Cri cal Volume (Eight- Phase Intersec on) Operang environment and ming objecves should be considered before a soware tool is selected. Signal Timing Manual, Second Edion

Chapter 5. Introducon to Timing Plans 5-27 5.3.1.1 Determinisc or Equaon-Based Models For many practitioners, deterministic or equation-based models are the models of choice when developing signal timing plans, particularly for coordinated systems. Deterministic or equation-based models are different than individual intersection methods (such as critical movement analysis) because they account for the arrival and departure of vehicles from one intersection to the next, otherwise known as the system effect. Trafic progression is treated explicitly through the use of time-space diagrams (explained in Chapter 7) or platoon progression techniques. Some models are also capable of deterministically estimating the effect of detection parameters for both vehicles and pedestrians. They typically use a combination of scaling factors for vehicle demand and the presence or absence of vehicle and/or pedestrian calls. A key feature of these signal timing models is the explicit effort to “optimize” signal timing to achieve a particular performance measure (such as delay), which may or may not tie into the desired outcome. To accomplish its optimization, each model uses some type of algorithm to test a variety of combinations of cycle length, splits, and offsets to achieve a calculated value of one or more performance indices, and then attempts to ind an optimal value for those performance indices. Most deterministic or equation- based models use elements of HCM procedures to estimate certain parameters, such as saturation low rate and delay. Saturation low rate is the hourly rate at which vehicles can traverse an intersection approach under prevailing conditions, assuming a constant green indication and no lost time. It is important to understand the appropriateness of the model as it relates to the operational objectives that the practitioner wants to achieve. If the operational objective is smooth arterial low with minimal stops, then the output from a delay minimization software tool may need to be manually adjusted to obtain values that are appropriate for the operational objective. For example, increasing the cycle length slightly may not correspond to the minimum possible delay (as determined by the software tool), but may signiicantly reduce the number of stops. These deterministic or equation-based models are frequently suficient for most signal timing applications. However, they can lose validity in cases where demand exceeds capacity or where the queues from one intersection interact with the operation of an adjacent intersection. In congested conditions, reducing the cycle length may actually increase throughput by reducing queues which block movements at other intersections. Therefore, locations with closely spaced intersections or with intersections exceeding capacity may not be well served by these types of deterministic models. In these cases, it may be necessary to use microscopic simulation models to obtain more realistic assessments of signal timing effects. 5.3.1.2 Microscopic Simulaon In the context of signal timing, microscopic simulation models can be thought of as an advanced evaluation tool. They can estimate car-following behaviors for vehicles and have the ability to model other users at a signalized intersection (including pedestrians, bicycles, and transit). Recent advances in technology also allow direct linkages between simulation models and either actual signal controllers or software emulations of those controllers, known as hardware-in-the-loop (HITL) and software-in-the-loop (SITL), respectively. These in-the-loop simulations allow actual controllers and/or their Determinisc or equaon-based models take mulple intersecons into account, unlike isolated intersecon methods. Signal Timing Manual, Second Edion

5-28 Chapter 5. Introduc on to Timing Plans algorithms to replace the approximation used in simulation models, more accurately re lecting how a controller will operate. The simulation model is used to generate traf ic lows and send vehicle and pedestrian calls to one or more controllers (based on the detection design implemented in the simulation). The controller receives the calls as if it was operating in the ield and uses its own internal algorithms to set signal indications based on the calls received and the implemented signal timing. The signal displays are passed back to the simulation model to which traf ic responds. This type of analysis is particularly effective for modeling special controller features such as transit signal priority and railroad preemption, because these features are controller irmware (logic in hardware) speci ic. The Trafic Analysis Toolbox (3) describes simulation models as particularly useful for the following applications: • To evaluate signals incorporating actuated-coordinated operations. The simulation program (if it includes realistic controller features and is coded correctly) may provide a more realistic assessment of the effectiveness of the actuated controller within the section being evaluated. • To con irm the likely presence of queue spillback between intersections. • To evaluate special features, such as transit priority (although generic transit signal priority models may not re lect the type of transit priority actually in use). • To evaluate system performance in the presence of saturated conditions. • To evaluate the effectiveness of manual adjustments to signal timing. • To evaluate fuel consumption and emissions or system travel time resulting from a given set of signal timing. • To evaluate various travel modes (i.e., pedestrian and bicycle traf ic). • To demonstrate improvements to public of icials, as they paint a clearer picture than numbers alone. This last application refers to the use of simulation models for their animated graphical outputs, which approximate aerial or 3-D views of the simulated network. These animated outputs permit visual assessment of system performance, a capability that cannot be duplicated by any other form of output, including time-space diagrams. Care must be used when reviewing animation from simulation models, as they should be re lective of an average run (which can be selected only after a review of the results). When running simulation analyses to compare alternatives, the practitioner should ensure that the resolution of the model is suf icient to model the type of control anticipated. A simulation model, for example, typically requires precision on the order of a tenth of a second to accurately replicate gap detection. Therefore, the use of coarser resolution settings (or a model with only 1-second resolution) to speed up simulation run time may yield incorrect results. 5.3.2 Soware Consideraons There are several items that should be considered when choosing which type of software tool to apply or whether to use a software application at all. The detector settings, saturated conditions of the area, user priority assumptions, unique network Microscopic simulaon models are advanced tools capable of approximang more signal ming controls than determinisc or equaon-based models. Signal Timing Manual, Second Edion

Chapter 5. Introducon to Timing Plans 5-29 features, and required ield calibration should be taken into account when choosing a software tool. 5.3.2.1 Detector Sengs Accurate detector locations, sizes, and timing parameters—often overlooked in software—are very important for creating accurate simulation results. A practitioner should determine the existing detector locations and settings (or plans for future implementation), as well as understand typical practice (i.e., standard detector layout plans) in order to build simulation models that are reliable. Software algorithms are subject to detector inputs, which can artiicially gap out early or extend unrealistically, creating inaccurate simulation results. Care should be taken to match detector locations, sizes, and timings accurately. 5.3.2.2 Saturated Condi ons Saturated conditions require careful distribution of green time that balances queue buildup and meters incoming demand. In addition, queued vehicles should be stored at locations where they will not impede other trafic. Because under-saturated low tends to be stable (similar from one cycle to the next), trafic signal timing software can often calculate optimal signal timing more easily under those conditions. However, many tools automatically assume under-saturated conditions, which can make evaluating oversaturated conditions dificult or produce erroneous results. As noted in the previous section, near, at, or oversaturated conditions may best be modeled by an experienced practitioner using a microscopic simulation model. Strategies related to oversaturated operations are discussed in Chapter 12. 5.3.2.3 User Priority Assump ons Many software tools make assumptions about user priorities without input from the practitioner. Review of user priorities should be completed when performing signal timing evaluations in order to assess whether the default software priorities match the practitioner-deined priorities. Adjustments may need to be made to the signal timing “optimized” by the software. 5.3.2.4 Unique Network Features Some unique network features, such as transit priority or a railroad, may warrant the use of a speciic software tool. Simple analysis techniques, such as critical movement analysis, will not be able to evaluate the effect of those types of features on vehicle progression or intersection capacity. 5.3.2.5 Field Observa on and Calibra on Field observations should be compared with trafic operation results for each time period in the model to validate that trafic volumes are correct. Then, if necessary, the base networks for each of the time periods should be calibrated using travel time, delay, and queue data collected from the ield. Parameters within the models that can be adjusted to calibrate the existing base networks with actual ield conditions include saturation low rates, right-turning-vehicles-on-red, and lane utilization. A review of the calibrated model should be performed prior to moving forward with the timing plan development. It should be noted, however, that ield observation is only a snapshot in A good rule of thumb is to have a sense for the expected answer (based on field knowledge or a quick cri cal movement analysis) to check if the so‡ware results are reasonable. Signal Timing Manual, Second Edion

5-30 Chapter 5. Introduc on to Timing Plans time. Modern controllers now have signi icant capabilities for assessing performance and these capabilities should not be overlooked in modeling operations. 5.3.3 Soware Inputs and Outputs Signal timing plan development typically concludes with the use of a software tool that helps determine appropriate cycle lengths, offsets, and splits for the intersections. In order to use a software tool to assist with the selection of such values, the practitioner irst needs to determine some of the basic signal timing parameters. Exhibit 5-31 provides a summary of the sections where typical software inputs and outputs are described in detail throughout this chapter and Chapters 6 and 7. Using the guidance from Chapter 6, a practitioner should be able to de ine most of the required software inputs. Chapter 7 provides additional information about the typical software outputs for a coordinated system (cycle lengths, offsets, and splits) and some information about the relationship between basic signal timing parameters and coordination. However, the practitioner should refer to software manuals for speci ic information about software operations. Signal Timing Parameter Chapter 5 Reference Chapter 6 Reference Chapter 7 Reference In pu ts Phase Sequence 5.1.1 Movement and Phase Numbering 5.1.2 Ring-and-Barrier Concept 5.1.3 Le-Turn Phasing 5.1.4 Overlaps 7.2.6 Bandwidth 7.6.1 Phase Sequence Minimum Green 6.1.3 Minimum Green Maximum Green 6.1.4 Maximum Green 7.4.3 Splits Guidance 7.4.4 Force-Offs Guidance Yellow Change 6.1.1 Yellow Change Red Clearance 6.1.2 Red Clearance Leading/Lagging Le Turns 5.1.3 Le­-Turn Phasing 7.2.6 Bandwidth 7.6.1 Phase Sequence Passage Time 6.1.5 Passage Time (Unit Extension or Gap Time) Minimum Gap 6.1.5 Passage Time (Unit Extension or Gap Time) Time Before Reduc on 6.1.5 Passage Time (Unit Extension or Gap Time) Time to Reduce 6.1.5 Passage Time (Unit Extension or Gap Time) Recalls 6.1.8 Recalls and Memory Modes Pedestrian Phasing 5.1.1 Movement and Phase Numbering Walk Interval 6.1.6 Pedestrian Intervals 7.5.1 Pedestrian Timing and Walk Modes Exhibit 5-31 References for Typical Soware Inputs and Outputs Signal Timing Manual, Second Edion

5-30 Chapter 5. Introduc on to Timing Plans time. Modern controllers now have signi icant capabilities for assessing performance and these capabilities should not be overlooked in modeling operations. 5.3.3 Soware Inputs and Outputs Signal timing plan development typically concludes with the use of a software tool that helps determine appropriate cycle lengths, offsets, and splits for the intersections. In order to use a software tool to assist with the selection of such values, the practitioner irst needs to determine some of the basic signal timing parameters. Exhibit 5-31 provides a summary of the sections where typical software inputs and outputs are described in detail throughout this chapter and Chapters 6 and 7. Using the guidance from Chapter 6, a practitioner should be able to de ine most of the required software inputs. Chapter 7 provides additional information about the typical software outputs for a coordinated system (cycle lengths, offsets, and splits) and some information about the relationship between basic signal timing parameters and coordination. However, the practitioner should refer to software manuals for speci ic information about software operations. Signal Timing Parameter Chapter 5 Reference Chapter 6 Reference Chapter 7 Reference In pu ts Phase Sequence 5.1.1 Movement and Phase Numbering 5.1.2 Ring-and-Barrier Concept 5.1.3 Le-Turn Phasing 5.1.4 Overlaps 7.2.6 Bandwidth 7.6.1 Phase Sequence Minimum Green 6.1.3 Minimum Green Maximum Green 6.1.4 Maximum Green 7.4.3 Splits Guidance 7.4.4 Force-Offs Guidance Yellow Change 6.1.1 Yellow Change Red Clearance 6.1.2 Red Clearance Leading/Lagging Le Turns 5.1.3 Le­-Turn Phasing 7.2.6 Bandwidth 7.6.1 Phase Sequence Passage Time 6.1.5 Passage Time (Unit Extension or Gap Time) Minimum Gap 6.1.5 Passage Time (Unit Extension or Gap Time) Time Before Reduc on 6.1.5 Passage Time (Unit Extension or Gap Time) Time to Reduce 6.1.5 Passage Time (Unit Extension or Gap Time) Recalls 6.1.8 Recalls and Memory Modes Pedestrian Phasing 5.1.1 Movement and Phase Numbering Walk Interval 6.1.6 Pedestrian Intervals 7.5.1 Pedestrian Timing and Walk Modes Exhibit 5-31 References for Typical Soware Inputs and Outputs Signal Timing Manual, Second Edion Chapter 5. Introducon to Timing Plans 5-31 Signal Timing Parameter Chapter 5 Reference Chapter 6 Reference Chapter 7 Reference In pu ts Flashing Don’t Walk Interval 6.1.6 Pedestrian Intervals 7.5.1 Pedestrian Timing and Walk Modes Dual Entry 6.1.7 Dual Entry Inhibit Max 7.4.3 Splits Guidance 7.4.4 Force-Offs Guidance Force-Offs 7.3.4 Force-Offs 7.4.4 Force-Offs Guidance Offset Reference Point 7.3.8 Offset Reference Point 7.4.8 Offset Reference Point Guidance Coordinated Phases 7.3.1 Coordinated Phases 7.4.1 Coordinated Phases Guidance Yield Point 7.3.6 Yield Point 7.4.6 Yield Point Guidance Detector Locaons and Sengs 5.1.5 Detector Assignments 6.2 Detector Configuraons 7.5.2 Actuang the Coordinated Phase O ut pu ts Cycle Length 7.3.2 Cycle Length 7.4.2 Cycle Length Guidance Splits 7.3.3 Splits 7.4.3 Splits Guidance Offsets 7.3.9 Offsets 7.4.9 Offsets Guidance 5.4 REFERENCES 1. Highway Capacity Manual 2010. Transportation Research Board of the National Academies, Washington, D.C., 2010. 2. Chandler, B. E., M. C. Myers, J. E. Atkinson, T. E. Bryer, R. Retting, J. Smithline, J. T. P. Wojtkiewicz, G. B. Thomas, S. P. Venglar, S. Sunkari, B. J. Malone, and P. Izadpanah. Signalized Intersections Informational Guide, Second Edition. Report FHWA-SA-13- 027, Federal Highway Administration, United States Department of Transportation, 2013. 3. Alexiadis, V., K. Jeannotte, and A. Chandra. Trafic Analysis Toolbox Volume I: Trafic Analysis Tools Primer. Report FHWA-HRT-04-038, Federal Highway Administration, United States Department of Transportation, 2004. Signal Timing Manual, Second Edion

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TRB’s National Cooperative Highway Research Program (NCHRP) Report 812: Signal Timing Manual - Second Edition, covers fundamentals and advanced concepts related to signal timing. The report addresses ways to develop a signal timing program based on the operating environment, users, user priorities by movement, and local operational objectives.

Advanced concepts covered in the report include the systems engineering process, adaptive signal control, preferential vehicle treatments, and timing strategies for over-saturated conditions, special events, and inclement weather.

An overview PowerPoint presentation accompanies the report.

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