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21st Century Operations Using 21st Century Technologies

Traffic Signal Timing Manual

CHAPTER 1

INTRODUCTION

TABLE OF CONTENTS

TABLE OF FIGURES

  • Figure 1-1 Organization of the Manual
  • Figure 1-2 User Interaction within the Signal Timing Process

1.0 INTRODUCTION

This Traffic Signal Timing Manual (TSTM) is intended to be a comprehensive guide to the traffic signal timing engineer and technician on traffic signal control logic principles, practices, and procedures. The TSTM represents a synthesis of traffic signal timing concepts, analytical procedures, and applications based on North American practice into a single publication. The manual also presents a framework for evaluating traffic signal timing applications related to maintenance and operations.

This manual is intended to complement policy documents such as the Manual on Uniform Traffic Control Devices, and is not intended to replicate or replace the Highway Capacity Manual, national or local engineering documents on signal timing, nor is it intended to serve as a standard or policy document. Rather, it provides a summary of practices intended to help practitioners in the timing of traffic signals.

1.1 BACKGROUND

The origin of traffic control signals can be traced back to the manually operated semaphores first used in London as early as 1868. The first traffic signal in the United States was developed with the objective to prevent accidents by alternatively assigning right of way. The traffic signal has changed significantly since its early development.

Today, there are more than 272,000 traffic signals in the United States (1). They play an important role in the transportation network and are a source for significant frustration for the public when not operated efficiently. As the era of freeway building draws to a close, urban arterials are being called upon to carry more users than ever before at a time when the users of these facilities are growing more complex (older drivers, more distractions, larger vehicles, etc) and the demand for such use continues to outpace transportation supply. According to the 2001 Nationwide Personal Transportation Survey, on average, an individual traveled 40 miles per day, up from approximately 35 in 1990 (2). At the same time, the use of traffic signals at a busy intersection in a typical urban area might direct the movement of as many as 100,000 vehicles per day. In fact over ten percent of all intersections in California carry more than 60,000 Average Daily Traffic (ADT) for movements (3). It is estimated that many of these signals could be improved by updating equipment or by simply adjusting and updating the timing plans. Outdated or poor traffic signal timing accounts for a significant portion of traffic delay on urban arterials and traffic signal retiming is one of the most cost effective ways to improve traffic flow and is one of the most basic strategies to help mitigate congestion.

Despite their important role in traffic management, traffic signals, once installed, are often not proactively managed. Maintenance activities are frequently delayed or canceled, in reaction to shrinking budgets and staffs. More than half of the signals in North America are in need of repair, replacement, or upgrading. In 2007, the National Traffic Signal Report Card was released by the National Transportation Operations Coalition and consisted of the composite national scores from an agency self-assessment related to traffic signal control and operations, the responses in five sub areas indicate an overall national "grade" of D up from a D- in 2005. (4).

FHWA has recognized the critical role that traffic signal timing plays within the overall transportation network. Signal timing offers the opportunity to improve the mobility and safety of the street system and contribute environmental benefits. This document is intended to further increase the awareness of the need for resources devoted to operation of the transportation system.

1.1.1 Purpose of Traffic Signals

The Manual on Uniform Traffic Control Devices (MUTCD) defines a traffic control signal as any highway traffic signal by which traffic is alternatively directed to stop and permitted to proceed. Traffic is defined as pedestrians, bicyclists, ridden or herded animals, vehicles, streetcars, and other conveyances either singularly or together while using any highway for purposes of travel. (5)

It is with this need to assign the right of way at locations that we consider the dual purpose of traffic signals —efficiency and safety— which in some cases seem to be conflicting. Safety may be seen as an element needed to be sacrificed in order to achieve improvements in efficiency and meet ever-increasing demands. The reality is that traffic signals can, and in fact must, serve both operational efficiency and safety based on the conditions. The MUTCD goes on to describe that traffic control signals can be ill-designed, ineffectively placed, improperly operated, or poorly maintained, with resulting outcomes of excessive delay, disobedience of the indication, avoidance, and increases in the frequency of collisions.

A traffic signal that is properly designed and timed can be expected to provide one or more of the following benefits:

  1. Provide for the orderly and efficient movement of people.
  2. Effectively maximize the volume movements served at the intersection.
  3. Reduce the frequency and severity of certain types of crashes.
  4. Provide appropriate levels of accessibility for pedestrians and side street traffic.

The degree to which these benefits are realized is based partly on the design and partly on the need for a signal. A poorly designed signal timing plan or an unneeded signal may make the intersection less efficient, less safe, or both.

1.1.2 Intersection Design and its Relationship to Signal Timing

The design of the intersection has a direct influence on its safety and operation from a design and user-ability perspective. Design elements that are particularly relevant include the number of lanes provided on each approach and for each movement, whether there are shared thru-and-turn lanes, the length of turn bays, the turning radii (especially important for pedestrians), the presence of additional through lanes in the vicinity of the intersection, the size and location of detectors, and presence or absence of left-turn phasing. Other geometric features, like additional through or turn lanes, can also have a significant positive impact on intersection capacity, provided that they are sufficiently long. The other aspect of intersection design is the perception and reaction of the end users. Various decisions need to be made as a user approaches the intersection, which makes it important to simplify the decision making process.

Another aspect of the design is detection. Detectors provide the ability to sense vehicle and pedestrian demands at an intersection; enabling modes of operation that may be more efficient than fixed or pre-timed control. It is critical that functional and properly designed detectors communicate with the controller to ensure continued functional signal control at the intersection. Detectors that are improperly located or are an inappropriate length can unnecessarily extend the green indication and increase the frequency of phase termination to the maximum limit (i.e., max out). Conversely, a poorly located detector could cause premature gap-out. A protected left-turn phase provides a time separation for left-turning and opposing traffic streams and may reduce left-turn delays or related crashes. However, the additional phase increases the minimum cycle length and may increase intersection delays and, in the case of a protected-only left-turn, may even increase left-turn delay.

The topics discussed in this section are intended to serve as a reminder of the close relationship between signal timing, intersection design, and traffic control device layout. The quality of the signal timing plan is directly tied to the adequacy of the intersection design and the traffic control device layout. In some situations, achieving safe and efficient intersection operation may require changes to the intersection design or the traffic control device layout. The subsequent chapters of this manual provide more detailed information about the role of these factors in signal timing plan development.

1.1.3 Objectives of Basic Signal Timing Parameters and Settings

A primary objective of signal timing settings is to move people through an intersection safely and efficiently. Achieving this objective requires a plan that allocates right-of-way to the various users. This plan should accommodate fluctuations in demand over the course of each day, week, and year.

Because travel demand patterns change over time, the signal timing plan should be periodically updated to maintain intersection safety and efficiency.

There are many signal timing parameters that affect intersection efficiency including the cycle length, movement green time, and clearance intervals. Increasing a traffic movement’s green time may reduce its delay and the number of vehicles that stop. However, an increase in one movement’s green time generally comes at the expense of increased delay and stops to another movement. Thus, a good signal timing plan is one that allocates time appropriately based on the demand at the intersection and keeps cycle lengths to a minimum.

The relationship between signal timing and safety is also addressed with specific timing parameters and the design of the intersection. For instance, the intent of the yellow change interval is to facilitate safe transfer or right-of-way from one movement to another. The safety benefit of this interval is most likely to be realized when its duration is consistent with the needs of drivers approaching the intersection at the onset of the yellow indication. This need relates to the driver’s ability to perceive the yellow indication and gauge their ability to stop before the stop line, or to travel through the intersection safely. Their decision to stop, or continue, is influenced by several factors, most notably speed. Appropriately timed yellow change intervals have been shown to reduce intersection crashes (6). Signal timing plans that reduce the number of stops and minimize delays may also provide some additional safety benefits.

The traffic signal controller at an intersection implements timing settings designed for that specific location. The settings are designed to respond to users at the intersection and meet objectives defined by the policies of the responsible agency.

The policies may include standards defined by the agency with potential guidance from regional or state standards and must consider pedestrians, vehicular traffic conditions, change and clearance intervals, and if actuated, detection layout. These settings may be influenced by adjacent intersections (the concept of coordination is more fully explored in Chapter 6), but are applicable for each intersection considered as an isolated unit.

1.1.4 Establishing the Need for Retiming

Traffic professionals have long recognized the value of designing effective signal timing to meet changing travel patterns and characteristics. In 1995, the U.S. General Accounting Office (GAO) reported, “Properly designed, operated and maintained traffic control signal systems yield significant benefits along the corridors and road networks on which they are installed. They mitigate congestion and reduce accidents, fuel consumption, air pollutants and travel times. Resource constraints have prevented the use of traffic signals to their full potential. The Traffic Signal Report Card Technical Report goes on to state:

“It became clear that for safety and liability reasons, agencies must ensure a basic level of operation of the traffic signal system so that signals continue to turn green, yellow and red. The signals may not function efficiently for traffic or pedestrians, but, technically, the signals are working and that is what people see. However, the uniformly low scores (on the National Report Card) indicate that, for the most part, people consistently experience poor traffic signal performance and, as a consequence, their expectations are low. The pattern, once again, is one where agencies are forced to use their resources to deal with critical maintenance issues when they arise rather than proactively. Signal systems are managed to simply ensure base levels of performance.”

The National Transportation Operations Coalition (NTOC) and FHWA continue to work to make the case that additional resources are needed to develop signal timing plans and to modernize 1-3 equipment. There’s an old saying that transportation engineers leave a little of their intelligence on the street when they design an intersection, but these designs are limited by the technology they have to work with. The use of 20-year-old technology and infrastructure may satisfy the requirement for the signal to display green, yellow, and red, but it may not offer the opportunity to efficiently operate the system or provide preferential treatment for a certain type of user to meet the policies and desires of the community. In most cases, upgraded equipment improves the efficiency for staff to manage the system, assuming the staff is properly trained to operate the upgraded equipment.

These efficiencies are observed with updating traffic signal timing plans, developing new strategies to improve transportation, and improving customer service. There have been some great technological advances in the past five years, such as the development of transit signal priority, which seeks to provide preferential treatment to buses as they approach the traffic signal. This new technology allows the engineer to allocate green time that more closely reflects the community’s transportation policies.

The MUTCD also speaks to the issue of efficiency and recommends proper design and signal timing to ensure that it satisfies current traffic demands. The Maintenance of signal timing plans will be described in Chapter 8 of this document. This statement does not speak to the need to provide preferential treatment or other policies that will be further described in Chapter 9 of this document.

1.1.5 Benefits of Up-to-Date Timing

Studies around the country have shown that the benefits of area-wide signal timing outweigh the costs 40:1 (or more). The benefits of up-to-date signal timing include shorter commute times, improved air quality, reduction in certain types and severity of crashes, and reduced driver frustration.(7)

The NTOC recently surveyed the quality of traffic signal operations in the United States. The NTOC concluded that the nation scored a D in terms of the overall quality of traffic signal operation. “If the nation supported its signals at an ‘A’ level, we would see:

  • Reductions in traffic delay ranging from 15-40% (8); reductions in travel time up to 25%; and reductions in stops ranging from 10-40% (9). For example, if you spent two hours in your car commuting to and from work and running errands, you’d save 50 hours per year (or more than a work week) because of improved signal timing.
  • Reductions in fuel consumption of up to 10%. Nationwide this would amount to a savings of almost 170 billion gallons of motor fuels per year.(10)
  • Reduction in harmful emissions (carbon monoxide, nitrogen oxides, volatile organic compounds) up to 22% (11). According to the Surface Transportation Policy Project, motor vehicles are the largest source of urban air pollution. (12) In addition, the EPA estimates that vehicles generate 3 billion pounds of air pollutants yearly.(13”,14)

Beyond the benefits to vehicular traffic, there are opportunities to improve performance for transit, pedestrians, and freight movement. Chapter 9 summarizes some of the advanced concepts that address some of the broad policies that have gained in popularity since the inception of the Intermodal Surface Transportation Efficiency Act of 1991 (15.)

1.2 ORGANIZATION OF THE MANUAL

The manual is organized into nine chapters that can be broadly described by four basic parts:

Part 1 —Policy, Planning, and Funding Considerations (Chapter 2). This part describes the need for and benefits of signal timing. It will present a discussion of relevant federal, state, regional, and local issues, as well as typical funding needs and options. This section discusses what decision makers consider when determining the effect of signal timing on the regional framework. It also presents how the signal timing process described in Chapter 7 fits into the regional planning framework.

Part 2— Fundamental Concepts of Capacity (Chapter 3) and Traffic Signal Design (Chapter 4). This part includes Chapters 3 and 4 which provide key background information needed to understand signal timing. This chapter may not be necessary for experienced engineers, but provides a basic foundation from which to describe more complicated concepts.

Part 3— Signal Timing Concepts, Guidelines, and Application and Coordination Plan Development (Chapters 5, 6, and 7). This part describes traffic signal timing from the concepts to application. It provides guidelines where appropriate based on industry practice. This is a chapter that provides several examples from agencies that represent good practice.

Part 4—Maintenance of Timing (Chapter 8) and Advanced Topics (Chapter 9). This part presents an overview of a number of advanced topics related to improving signal timing operations that will be especially relevant to sophisticated timing engineers that are implementing innovative strategies (transit signal priority, adaptive signal timing, etc). This section will provide an overview of each topic with references to more detailed documents.

This organization is described in Figure 1-1.

Figure 1-1 Organization of the Manual

The diagram in Figure 1-1 illustrates the four part organization of the manual. Part one encompassing chapter two, consists of Policy, Planning, and Funding Considerations. Part two, consisting of Chapter 3 and 4, reviews the fundamentals of capacity and signal design. Part three, encompassing chapters 5 through 7, covers concepts of basic and coordinated signal timing, and the process for developing coordination plans.  Part 4 of the manual covers maintenance of timing and other advanced topics.

1.3 USE OF THE MANUAL

The Traffic Signal Timing Manual (TSTM) is intended to be a comprehensive document describing the procedures for generating signal timing plans for North American applications. It is intended for use by a range of practitioners; including traffic engineers, signal technicians, design engineers, planners, transportation managers, teachers, and university students. To use the manual effectively and apply its methodologies, some technical background is desirable, typically technical training either provided as a part of continuing education, or at the university-level.

To help describe the use of this manual, three generalized users of the signal timing manual have been identified, planners, designers, and operators/maintenance staff. Each of these users will interact with different elements of the signal timing process, and should understand where and how the others are involved. Figure 1-2 shows graphically the interaction or relationship between the generalized user function: planning, design, and operations/maintenance, and the key topics of signal timing, shown in white.

Figure 1-2 User Interaction within the Signal Timing Process

The diagram in Figure 1-2 illustrates three functions of planning, design, and operations/maintenance and their roles and responsibilities in relation to the key topics of traffic signal timing.  The paragraph that follows briefly describes the key topics.

The general topics shown in Figure 1-2, represent the basic signal timing process, and are described in detail within the manual. The topics are described briefly here:

  • Signal Timing Project Initiation– Establish the purpose(s) of the signal timing project and how success will be measured. This will encompass all three types of users. 1-6
  • User of the Signal – Determine and prioritize the users on the transportation network that the signal must serve. For example, motor vehicles, pedestrians, bicycles, transit, emergency vehicles, etc.
  • Geometry – Determine the lane use, dedication, and geometry and roadway infrastructure with which the signal timing will interact. Roadway infrastructure may not change, but lane designation and use can change along with signal timing parameters. For example, multiple turn-lanes, freeway ramp terminal, channelized movements, turn-restrictions, phasing, etc.
  • Phasing – Set the appropriate signal phasing scheme(s). For example, split phasing, protected left-turns, overlapping movements, etc.
  • Detector Placement – Determine if and where detectors are needed based on desired signal operations. For example, pedestrian push buttons or bike/vehicle detection locations, or even transit priority or emergency vehicle preemption detection zones.
  • Detector Function – Set the functionality of the detectors. For example, vehicle detector settings of pulse or presence or count, or a delay setting for movements with heavy right turn on red, or etc.
  • Basic Signal Timing – Calculate or establish the appropriate basic timing parameters. For example, pedestrian walk and flash don’t walk, gap or passage time, yellow and all red vehicle clearance, etc.
  • Mode of Operation – Will the signal operate in fixed-time, actuated, or adaptive mode?
  • Coordination Plan – Determine if the signal should be coordinated with other nearby signals as a system or operate in isolation? Coordinating will require interconnection communication lines between traffic signal controller cabinets.
  • Performance Measurement – What measurements will be used to judge the performance of the signal timing and operations? Typically performance measurements are average delay, travel time, stops, and etc.

Each of these steps may take place in the planning, design or operation of a traffic signal and its signal timing plans. To help understand this interrelationship, let’s take the example of detection placement. From Figure 1-2, we see this area involves both design and operations/maintenance. A designer may select a inductive loop detector placement location based on a design manual or agency policy for the design travel speed. What the designer may not realize is once the loop detection is embedded in the pavement, it is difficult to relocate. If the operator or traffic engineer wants to change the posted speed or operate the signal using detection intensive, adaptive control, alternate loop locations maybe necessary. This would force the costly relocation of the designed loops. If the designer and operator/engineer communicate and work together to understand each others needs, a multi-use solution can be developed to benefit the roadway user.

Many of the topics discussed in the Traffic Signal Timing Manual will be applicable to multiple types of users, and an understanding of signal timing holistically will allow the best solutions possible to be developed and implemented, to the gain of the traveling public.

1.4 REFERENCES

  1. 2007 National Traffic Signal Report Card Technical Report, National Transportation Operations Coalition.
  2. USDOT. Nationwide Personal Transportation Survey, http://nhts.ornl.gov/2001/index.shtml, 2001.
  3. Highway Safety Information System (HSIS). California database, 1994-1998.
  4. National Transportation Operations Coalition, National Traffic Signal Report Card Technical Report, 2007.
  5. Federal Highway Administration (FHWA). Manual of Uniform Traffic Control Devices. Washington, DC: USDOT, FHWA, 2003.
  6. Retting, R.A.; Chapline, J.F.; and Williams, A.F., “Changes In Crash Risk Following Re-Timing Of Traffic Signal Change Intervals”. Accident Analysis and Prevention 34:215-20, 2002
  7. Ibid.
  8. “Temporary Losses of Highway Capacity and Impacts on Performance: Phase 2, Oakridge National Laboratory, November 2004, ORNL/TM-2004/209 and “Benefits of Retiming Traffic Signals”, ITE draft Informational Report, 2005.
  9. “Benefits of Retiming Traffic Signals”, ITE draft Informational Report, 2005.
    http://www.bts.gov/publications/national_transportation_statistics/2004/html/table_04_09.html
  10. ITS Benefits and Costs Database, USDOT ITS Joint Program Office, http://www.benefitcost.its.dot.gov/
  11. Surface Transportation Policy Project. http://www.transact.org/library/factsheets/environment.asp.
  12. National Toxics Inventory, 1996. http://www.epa.gov/otaq/regs/toxics/d00003.pdf
  13. Ibid “National Traffic Signal Report Card”
  14. “ISTEA Reauthorization Policy Statement And Principles”, U.S. Department of Transportation, http://www.dot.gov/ost/govtaffairs/istea/isteap&p.html January 6, 2007

 

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