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FHWA Home / Safety / Intersection / Signalized Intersections: An Informational Guide

Signalized Intersections: An Informational Guide

Chapter 1. Introduction

Table of Contents

1.0 Introduction
1.1 Background
1.2 Performance Measurement and Asset Management
1.3 Scope of the Guide
1.4 Audience for this Guide
1.5 Organization of the Guide

List of Exhibits

1-1 Summary of motor vehicle crashes related to junction and severity in the United States during 2009
1-2 Vehicle detection by inductive loop (left) and video (right)
1-3 Inside a signal controller cabinet
1-4 Signalized intersection with four approaches
1-5 Organization of the guide
1-6 List of intersection treatments discussed in this guide

1.0 Introduction

References to be used throughout the Guide include:
  • TRB Highway Capacity Manual (2010)
  • FHWA Manual on Uniform Traffic Control Devices (2009)
  • AASHTO A Policy on Geometric Design of Highways and Streets (2011)
  • AASHTO Highway Safety Manual (2010)
  • TRB NCHRP Report 500 series
  • FHWA Alternative Intersections and Interchanges Informational Report (2010)
  • U.S. Access Board Draft Public Rights-of-Way Accessibility Guidelines (2011)
  • U.S. Access Board ADAAG Requirements for Detectable Warnings (2008)
  • U.S. Access Board ADA Standards for Accessible Design (2010)
  • FHWA Traffic Signal Timing Manual (2008)
  • FHWA Traffic Detector Handbook (2006)
  • ITE Traffic Control Devices Handbook (2011)
  • IESNA American National Standard Practice for Roadway Lighting (2005)

This document serves as an introduction to and guide for evaluating the safety, design and operations of signalized intersections and provides tools to deliver better balanced solutions for all users. The treatments in this guide range from low-cost measures such as improvements to signal timing and signing, to high-cost measures such as intersection reconstruction or grade separation. While some treatments apply only to higher volume intersections, much of this guide is applicable to signalized intersections of all volume levels.

The guide takes a holistic approach to signalized intersections and considers the safety and operational implications of a particular treatment on all system users (e.g., motorists, pedestrians, bicyclists, and transit users). When applying operational or safety treatments, it is often necessary to consider the impact one will have on the other. This guide will introduce the user to these trade-offs and their respective considerations.

Practitioners will find the tools and information necessary to make insightful intersection assessments and to understand the impacts of potential improvement measures. The information in this guide is based on the latest research available and includes examples of novel treatments as well as best practices in use by jurisdictions across the United States and other countries. Additional resources and references are mentioned for the practitioner who wishes to learn more about a particular subject.

This guide is not intended to replicate or replace traditional traffic engineering documents such as the Manual on Uniform Traffic Control Devices (MUTCD),(1) the Highway Capacity Manual (HCM) 2010 (2) or the American Association of State Highway and Transportation Officials' (AASHTO) A Policy on Geometric Design of Highways and Streets,(3) nor is it intended to serve as a standard or policy document. Rather, it provides a synthesis of best practices and treatments intended to help practitioners make informed, thoughtful decisions.

1.1 Background

Traffic Signal Basics

Traffic signals are electrically operated traffic control devices that provide indication for roadway users to advance their travels by assigning right-of-way to each approach and movement. Traffic signals are a common form of traffic control used by State and local agencies to address roadway operations and safety issues. They allow the shared use of road space by separating conflicting movements in time and allocating delay, and can be used to enhance the mobility and safety of some movements.

The installation of traffic signals may be considered when any of the following conditions are desired:

The installation of traffic signals is determined by analyzing traffic volume data, crash history, roadway geometry, and field conditions. Planners, designers, and traffic engineers work together to determine if conditions are right for installation. Several safety and mobility factors should be considered as new traffic signal installation is being discussed. Chapter 4C of the MUTCD outlines basic warrants for when installation of a traffic signal may be justified. In addition to the considerations presented in the MUTCD, practitioners should give thought to roadway/intersection geometry and sight distance, driver expectancy, and locations of nearby traffic signals when considering the installation of new traffic signals.

When weighing the options for traffic control types at an intersection, there are several important factors to consider:

Once installed, traffic signals are operated and maintained by traffic engineers and field traffic signal technicians. Those who are responsible for maintenance and operations should regularly perform site visits to:

Ideally, field traffic signal technicians are qualified to perform maintenance inspections at regular intervals and repairs are made such that the signal operates safely and efficiently at all times. Technicians are also responsible for the general upkeep and operation of signal equipment located at the intersection.

At some point during its life, a traffic signal will be identified for upgrades, replacement, or decommission. An upgrade or full replacement may be needed due to degradation of equipment, newer technologies being available, or changing conditions at the site such as lane additions or the need for alternate phasing. In some instances, the traffic signal may be completely removed if traffic patterns cease to warrant its use.

Traffic Operations: Safety and Mobility

Traffic signals play a prominent role in achieving safer performance at intersections. Research has shown that under the right circumstances, the proper installation and operation of traffic signals will reduce the severity of crashes. But the addition of unnecessary signals or signals that are inappropriately designed and/or located can have an adverse effect on traffic safety and mobility. Care in their placement, design, and operation is essential.

In some cases, the dual objectives of mobility and safety conflict. To meet increasing and changing demands, one element may need to be sacrificed to some degree to achieve improvements in another. In all cases, it is important to understand the degree to which traffic signals are providing mobility and safety for all roadway users.

Assuring the efficient operation of the traffic signal is becoming an increasingly important issue as agencies attempt to maximize vehicle roadway capacity to serve the growing demand for travel, while maintaining a high level of safety.

Reducing crashes should always be one of the objectives whenever the design or operational characteristics of a signalized intersection are modified. As described by the Federal Highway Administration (FHWA), the "mission is not simply to improve mobility and productivity, but to ensure that improved mobility and productivity come with improved safety."(4)

Exhibit 1-1 shows that in 2009, 21 percent of all crashes and 24 percent of all fatalities and injury collisions occurred at signalized intersections.

Exhibit 1-1. Summary of motor vehicle crashes related to junction and severity in the United States during 2009.
  Total Crashes Fatalities/Injuries
Number Percent Number Percent
Non-Intersection Crashes 3.295,000 60 841,027 54
Signalized Intersection Crashes 1,158,000 21 372,299 24
Non-Signalized Intersection Crashes 1,052,000 19 332,471 22
Total 5,505,000 100 1,547,797 100
Source: Adapted from table 29 of Traffic Safety Facts 2009.(5)

How a Traffic Signal Works

Traffic signals are designed to allow for the safe and efficient passage of road users when demand exists. Types of operation include pre-timed, semi-actuated, fully-actuated, hybrid, adaptive, or traffic responsive. Pre-timed signals are programmed to give right-of-way to movements based on a predetermined allocation of time. Semi-actuated signals use various detection methods to identify roadway users on the minor approaches, while fully-actuated signals recognize users on all approaches. Chapter 5 will discuss each of these methods in further detail.

Traffic signals are comprised of several hardware components, and depending on the actuation method, may have additional equipment. The following paragraphs provide information related to each component.

Exhibit 1-2. Vehicle detection by inductive loop (left) and video (right)
Side-by-side photos of a loop detector in pavement and a camera mounted to signal arm.
(Source: Left: South Carolina DOT / Right: Jeff Shaw, FHWA)

Detection.

Road users can be sensed using various methods of detection at semi- and fully-actuated signals. Detection methods for motorists include in-pavement loop detectors or sensors (Exhibit 1-2 (left)) and cameras mounted to signal poles (Exhibit 1-2 (right)), and detection methods for pedestrians and bicyclists include push button and weight sensors.

Traffic signal controller.

Each detection method sends vehicle presence information to a traffic signal controller. The controller acts as the "brain" of the traffic signal, changing signal indications based on user needs. The controller will determine when the indication for the approach will change and how much time will be given to each movement. A controller is shown in Exhibit 1-3.

Algorithms used for determining the priority and length of time of each approach movement are tailored to the needs of each intersection. The algorithm is based on historical user demand, crash history, and other roadway network considerations.

Exhibit 1-3. Inside a signal controller cabinet.
The inside of a signal controller cabinet.
Photo Credit: Missouri DOT

Signal heads.

Traffic signal heads inform roadway users of when their movement can proceed through the intersection. Signal heads for motorists and bicyclists are usually mounted on mast arms or span wire above the travel lane, and sometimes repeated on the signal pole. Pedestrian signal heads are often installed on the traffic signal pole, or can be installed independently on separate poles depending on the intersection design. Signal heads vary in configuration, shape, and size depending on the movement for which they are used.

Types of Signalized Intersections

In their most common form, signalized intersections have indications for users on each intersection approach. A basic signalized intersection with four approaches is shown in Exhibit 1-4. The figure shows two signalized pedestrian approaches.

In addition to signalizing intersections, it may be necessary to consider the use of pedestrian signals at locations along a corridor with high concentrations of pedestrians. This type of traffic control can be used at signalized intersections with the addition of pedestrian push-buttons and signal heads, or at non-signalized locations that have high volumes of pedestrians crossing. This guide also provides direction on the use of treatments such as the Pedestrian Hybrid Beacon. Pedestrian signals are discussed in more detail in Chapter 5.

1.2 Performance Measurement and Asset Management

Agencies are faced with providing outstanding customer service with limited resources. Performance measures allow practitioners to assess the effectiveness of a signalized intersection or corridor. These measures can help agencies allocate resources to make adjustments. Criteria related to travel could be stopped delay, travel speed, arrivals on red, or excessive queuing. Criteria related to safety could be crash frequency, crash types, and severity. Information connected to repair history could be categorized according to time of day or types of repair. These measures can be adjusted by the practitioner and agency as deemed necessary.

The practitioner should review this data to assess problem areas to correct. Other information that may be needed includes comments from the practitioner's annual signal timing reviews and annual preventive maintenance program. Examples of questions that may arise from such a review:

Practitioners should create queries that identify problematic intersections. These queries can also identify global intersection treatments that reduce systematic problems. For example, an agency could choose to install uninterrupted power supply (UPS) units for frequent power outages. The following is a list of information that could be utilized to monitor performance:

Reviews of these measures should involve traffic engineers, technicians, and operations personnel to create a culture of continuous improvement.

1.3 Scope of the Guide

This guide addresses safety and operation for all users of signalized intersections, including motorists, pedestrians, bicyclists, and transit riders. This guide addresses Americans with Disabilities Act (ADA) requirements and provides guidelines for considering older drivers.

Roundabouts and other alternative intersection designs are not addressed directly in this document; for more information, please refer to Roundabouts: An Informational Guide, Second Edition (6) and the FHWA Alternative Intersections/Interchanges Informational Report.(55)

1.4 Audience for this Guide

This guide is intended for planners, designers, traffic engineers, operations analysts, and signal technicians who perform or want to perform one or more of the following functions as they pertain to signalized intersections:

It is envisioned that this guide will be used by signal technicians, design and traffic engineers, planners, and decision-makers who:

Exhibit 1-4. Signalized intersection with four approaches.
Aerial photo of a signalized intersection with four approaches.
Source: FHWA

1.5 Organization of the Guide

This guide is arranged in three parts:

The chapters on fundamentals (Chapters 2-5) in Part I provide key background information on three topic areas: user needs, data collection, signal warrants, geometric design, and traffic design and illumination. These chapters provide a foundation of knowledge of signalized intersections that is useful as a learning tool for entry-level engineers and as a refresher for more experienced engineers. The information contained in these chapters is referenced in Parts II and III.

The chapters on project process and analysis methods (Chapters 6-7) in Part II outline the steps that should be carried out and the tools to consider for evaluating the safety and operational performance of an intersection and determining geometric and timing needs.

Part III provides a description of treatments that can be applied to mitigate a known safety or operational deficiency. The treatments are organized in Chapters 8-11 based on the intersection element. Within each chapter, the treatments are grouped by a particular user type (e.g., pedestrian treatments) or are grouped to reflect a particular condition (e.g., signal head visibility).

Exhibit 1-5 depicts the organization of the guide.

Exhibit 1-5. Organization of the guide.
Part Chapter Title
  1 Introduction
Part I: Fundamentals 2 User Needs
3 Data Collection and Warrants
4 Geometric Design
5 Traffic Design and Illumination
Part II: Project Process and Analysis Methods 6 Safety Analysis Methods
7 Operational Analysis Methods
Part III: Treatments 8 System-Wide Treatments
9Intersection-Wide Treatments
10 Approach Treatments
11 Individual Movement Treatments

Exhibit 1-6 provides a list of the treatments discussed in Part III. Each treatment includes a description, a photo or diagram where available, and a summary of the treatment's applicability. In addition, these sections identify key design elements; operational and safety impacts; impacts on other modes; socioeconomic and physical impacts; and education, enforcement, and maintenance issues. The treatments in Exhibit 1-6 represent some, but not all, possible treatments.

Exhibit 1-6. List of intersection treatments discussed in this guide.
Treatment Type Treatments
System-Wide Treatments (Chapter 8)
  • Median treatments
  • Access management
  • Provide signal coordination
  • Provide signal preemption/priority
Intersection-Wide Treatments (Chapter 9)
  • Reduce curb radius
  • Provide curb extensions
  • Modify stop line location
  • Improve pedestrian signal display
  • Modify pedestrian signal phasing
  • Grade separate pedestrian movements
  • High visibility crosswalks
  • Provide bicycle box (experimental)
  • Provide bike lanes
  • Relocate transit stop
  • Change signal control from pre-timed to actuated
  • Modify change and clearance intervals
  • Modify cycle length
  • Remove late night/early morning flash
  • Provide or upgrade illumination
  • Convert signalized intersection to a roundabout or all-way stop control
Approach Treatments (Chapter 10)
  • Convert to over-the-road signal heads
  • Add supplemental signal heads
  • Increase size of signal heads
  • Increase number of signal heads
  • Provide backplates
  • Provide advance warning
  • Improve lane use and street name signing
  • Reduce operating speed
  • Improve pavement surface
  • Improve cross section
  • Remove obstacles from clear zone
  • Improve sight lines
  • Provide dilemma zone protection
  • Provide red light camera enforcement
Individual Movement Treatments (Chapter 11)
  • Add single left-turn lane
  • Add multiple left-turn lane
  • Add channelizing islands
  • Add single right-turn lane
  • Provide double right-turn lanes
  • Restrict turns, U-turns
  • Provide auxiliary through lane
  • Delineate through path
  • Provide reversible lane
  • Provide variable lane use assignments

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Page last modified on September 4, 2014.
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