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Publication Number: FHWA-HRT-04-091
Date: August 2004
Signalized Intersections: Informational Guide
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CHAPTER 1 — INTRODUCTIONTABLE OF CONTENTS
LIST OF TABLES
This guide provides a single, comprehensive document with methods for evaluating the safety and operations of signalized intersections and tools to remedy deficiencies. The treatments in this guide range from low-cost measures such as improvements to signal timing and signage, 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 (motorists, pedestrians, bicyclists, transit users).
Practitioners will find the tools and information necessary to make insightful intersection assessments and to understand the tradeoffs of potential improvement measures. The information here 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. Additional resources and references are highlighted 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) 2000,(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 the best practices and novel treatments intended to help practitioners make informed, thoughtful decisions.
Traffic signals are a common form of traffic control used by State and local agencies to address roadway operations. They allow the shared use of road space by separating conflicting movements in time and allocating delay. They can also be used to enhance the mobility of some movements as, for example, along a major arterial.
Traffic signals play a prominent role in achieving safer performance at intersections. Research has shown that, under the right circumstances, the installation of traffic signals will reduce the number and severity of crashes. But inappropriately designed and/or located signals can have an adverse effect on traffic safety, so care in their placement, design, and operation is essential. Table 1 shows that in 2002, 21 percent of all crashes and 24 percent of all fatalities and injury collisions occurred at signalized intersections.
Table 1 . Summary of motor vehicle crashes related to junction and severity in the United States during 2002.
Source: Adapted from table 28 of Traffic Safety Facts 2002.(4)
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 each of transportation.
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. "Quick fixes" and low-cost treatments are increasingly limited.
Grade separation has traditionally been viewed as the next logical step beyond a signalized intersection. In some cases, grade separation may be the most appropriate choice, and the practitioner needs guidance on when and how to make the transition to an interchange. However, given the construction costs, availability of right-of-way, and social and environmental constraints associated with a grade-separated improvement, alternative traffic control and geometric design forms are sometimes preferable if they can be shown to be feasible and adequate from operational, safety, and design perspectives.
A variety of alternative traffic control forms can be found around the country, many of which have seen widespread application in only limited geographic areas. For example, New Jersey has long had a practice of using jughandle left turns to improve the operation of signalized intersections. Michigan has used median U-turns to eliminate movements at critical intersections. Maryland and New York have constructed versions of a continuous flow intersection.
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."(5)
1.2 Scope of Guide
This guide covers all aspects of signalized intersections, with some emphasis on signalized intersections with traffic volumes typically exceeding 40,000 entering vehicles per day where all approaches are heavily used. These are intersections well beyond MUTCD volume thresholds for signal warrants. This is intended to cover arterial/arterial level facilities and higher. Intersections of this size typically include a minimum of five lanes on the major street and three lanes on the minor street. Intersections considered for grade separation are within the scope of the guide.
The intersection forms considered here include signalized at-grade intersections and intersections with the potential for grade separation. Intersections that include alternative or unconventional turn treatments such as median U-turns and jughandles are within the scope of these guidelines to the extent that they have been implemented and accepted by some U.S. jurisdictions. Roundabouts are not addressed in this document; for more information, please refer to FHWA's Roundabouts: An informational Guide.(6)
The 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.
1.3 AUDIENCE FOR This Guide
This guide is intended for planners, designers, and operations analysts 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 engineers, planners, and decisionmakers who:
1.4 Organization of the Guidelines
This guide is arranged in three parts:
The chapters on fundamentals (chapters 2-4) in part i provide key background information on three topic areas: user needs, 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 5-7) in part II outline the steps that should be carried out in a project involving the evaluation and assessment of a signalized intersection. Part II also provides tools practitioners can use to evaluate the safety and operational performance of an intersection and determine geometric 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 8-12 based on the intersection element (system, approach, movement, etc.). 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).
Table 2. Organization of the guide.
Table 3 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 table 3 represent some, but not all, possible treatments.
Table 3. List of intersection treatments discussed in this document.
Part I — Fundamentals
Part I discusses the fundamentals of signalized intersections as they relate to User Needs (chapter 2), Geometric Design (chapter 3), and traffic Design and Illumination (chapter 4). These chapters are intended for use by entry-level engineers and other users of the guide who seek broad-level information on the technical aspects of signalized intersections. The information provides a background for the chapters in part II and part III.
Keywords: Signalized Intersections, Intersection Safety, Intersection Design, Intersection Performance, Intersection Treatments
TRT Terms: Roads–Interchanges and intersections, Roads–Interchanges and intersections–Safety measures, Traffic signs and signals, Signalized intersections, State of the art reports, Best practices