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The electronic version of this document was developed by: Mr. Ivan Lorenz and Mr. Daniyar Satarov, Texas Transportation Institute, College Station, Texas

SPINE = 5/16"Job No._NCHRP#457 NCHRP Green

TRANSPORTATION RESEARCH BOARD

Evaluating IntersectionImprovements:

An Engineering Study Guide

NATIONALCOOPERATIVE HIGHWAYRESEARCH PROGRAMNCHRP

REPORT 457

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eport 457Evaluating Intersection Im

provements: An Engineering Study Guide

Program Staff

ROBERT J. REILLY, Director, Cooperative Research ProgramsCRAWFORD F. JENCKS, Manager, NCHRPDAVID B. BEAL, Senior Program OfficerHARVEY BERLIN, Senior Program OfficerB. RAY DERR, Senior Program OfficerAMIR N. HANNA, Senior Program OfficerEDWARD T. HARRIGAN, Senior Program Officer

TIMOTHY G. HESS, Senior Program OfficerRONALD D. McCREADY,

CHARLES W. NIESSNER, Senior Program Officer

Senior Program Officer

EILEEN P. DELANEY, Managing EditorJAMIE FEAR, Associate EditorHILARY FREER, Associate EditorANDREA BRIERE, Assistant EditorBETH HATCH, Editorial AssistantCHRISTOPHER HEDGES, Senior Program Officer

TRANSPORTATION RESEARCH BOARD EXECUTIVE COMMITTEE 2001

OFFICERSChair: John M. Samuels, Senior Vice President-Operations Planning & Support, Norfolk Southern Corporation, Norfolk, VAVice Chair: Thomas R. Warne, Executive Director, Utah DOT Executive Director: Robert E. Skinner, Jr., Transportation Research Board

MEMBERSWILLIAM D. ANKNER, Director, Rhode Island DOTTHOMAS F. BARRY, JR., Secretary of Transportation, Florida DOTJACK E. BUFFINGTON, Associate Director and Research Professor, Mack-Blackwell National Rural Transportation Study Center, University of ArkansasSARAH C. CAMPBELL, President, TransManagement, Inc., Washington, DCE. DEAN CARLSON, Secretary of Transportation, Kansas DOTJOANNE F. CASEY, President, Intermodal Association of North AmericaJAMES C. CODELL III, Transportation Secretary, Transportation Cabinet, Frankfort, KYJOHN L. CRAIG, Director, Nebraska Department of RoadsROBERT A. FROSCH, Senior Research Fellow, John F. Kennedy School of Government, Harvard UniversityGORMAN GILBERT, Director, Oklahoma Transportation Center, Oklahoma State UniversityGENEVIEVE GIULIANO, Professor, School of Policy, Planning, and Development, University of Southern California, Los AngelesLESTER A. HOEL, L. A. Lacy Distinguished Professor, Department of Civil Engineering, University of VirginiaH. THOMAS KORNEGAY, Executive Director, Port of Houston AuthorityBRADLEY L. MALLORY, Secretary of Transportation, Pennsylvania DOTMICHAEL D. MEYER, Professor, School of Civil and Environmental Engineering, Georgia Institute of TechnologyJEFFREY R. MORELAND, Executive Vice President-Law and Chief of Staff, Burlington Northern Santa Fe Corporation, Fort Worth, TXSID MORRISON, Secretary of Transportation, Washington State DOTJOHN P. POORMAN, Staff Director, Capital District Transportation Committee, Albany, NYCATHERINE L. ROSS, Executive Director, Georgia Regional Transportation AgencyWAYNE SHACKELFORD, Senior Vice President, Gresham Smith & Partners, Alpharetta, GAPAUL P. SKOUTELAS, CEO, Port Authority of Allegheny County, Pittsburgh, PAMICHAEL S. TOWNES, Executive Director, Transportation District Commission of Hampton Roads, Hampton, VAMARTIN WACHS, Director, Institute of Transportation Studies, University of California at BerkeleyMICHAEL W. WICKHAM, Chairman and CEO, Roadway Express, Inc., Akron, OHJAMES A. WILDING, President and CEO, Metropolitan Washington Airports AuthorityM. GORDON WOLMAN, Professor of Geography and Environmental Engineering, The Johns Hopkins University

MIKE ACOTT, President, National Asphalt Pavement Association (ex officio)EDWARD A. BRIGHAM, Acting Deputy Administrator, Research and Special Programs Administration, U.S.DOT (ex officio)BRUCE J. CARLTON, Acting Deputy Administrator, Maritime Administration, U.S.DOT (ex officio)JULIE A. CIRILLO, Assistant Administrator and Chief Safety Officer, Federal Motor Carrier Safety Administration, U.S.DOT (ex officio)SUSAN M. COUGHLIN, Director and COO, The American Trucking Associations Foundation, Inc. (ex officio)ROBERT B. FLOWERS (Lt. Gen., U.S. Army), Chief of Engineers and Commander, U.S. Army Corps of Engineers (ex officio)HAROLD K. FORSEN, Foreign Secretary, National Academy of Engineering (ex officio)JANE F. GARVEY, Federal Aviation Administrator, U.S.DOT (ex officio)EDWARD R. HAMBERGER, President and CEO, Association of American Railroads (ex officio)JOHN C. HORSLEY, Executive Director, American Association of State Highway and Transportation Officials (ex officio)S. MARK LINDSEY, Acting Deputy Administrator, Federal Railroad Administration, U.S.DOT (ex officio)JAMES M. LOY (Adm., U.S. Coast Guard), Commandant, U.S. Coast Guard (ex officio)WILLIAM W. MILLAR, President, American Public Transportation Association (ex officio)MARGO T. OGE, Director, Office of Transportation and Air Quality, U.S. Environmental Protection Agency (ex officio)VALENTIN J. RIVA, President and CEO, American Concrete Pavement Association (ex officio)VINCENT F. SCHIMMOLLER, Deputy Executive Director, Federal Highway Administration, U.S.DOT (ex officio)ASHISH K. SEN, Director, Bureau of Transportation Statistics, U.S.DOT (ex officio)L. ROBERT SHELTON III, Executive Director, National Highway Traffic Safety Administration, U.S.DOT (ex officio)MICHAEL R. THOMAS, Applications Division Director, Office of Earth Sciences Enterprise, National Aeronautics Space Administration (ex officio)HIRAM J. WALKER, Acting Deputy Administrator, Federal Transit Administration, U.S.DOT (ex officio)

NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAMTransportation Research Board Executive Committee Subcommittee for NCHRP

ROBERT E. SKINNER, JR., Transportation Research BoardMARTIN WACHS, Institute of Transportation Studies, University of California at

BerkeleyTHOMAS R. WARNE, Utah DOT

JOHN M. SAMUELS, Norfolk Southern Corporation, Norfolk, VA (Chair)LESTER A. HOEL, University of VirginiaJOHN C. HORSLEY, American Association of State Highway and Transportation

OfficialsVINCENT F. SCHIMMOLLER, Federal Highway Administration

ROBERT H. WORTMAN, University of Arizona (Chair)

RICHARD E. BENNETT, Missouri DOT

THOMAS H. CULPEPPER, Wilbur Smith Associates, Falls Church, VA

BETH P. DE ANGELO, Parsons Brinckerhoff-FG, Inc., Princeton, NJ

GLENN M. GRIGG, Sunnyvale, CA

PARA M. JAYASINGHE, City of Boston Public Works Department

STEVEN G. JEWELL, City of Columbus Traffic Engineering Division, Columbus, OH

KEN KOBETSKY, AASHTO

PAWAN MAINI, Fitzgerald & Halliday, Inc., Midlothian, VA

HENRY LIEU, FHWA Liaison Representative

RICHARD A. CUNARD, TRB Liaison Representative

Project Panel G3-58 Field of Traffic Area of Operations and Control

T R A N S P O R T A T I O N R E S E A R C H B O A R D — N A T I O N A L R E S E A R C H C O U N C I L

NATIONAL ACADEMY PRESSWASHINGTON, D.C. — 2001

NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM

NCHRP REPORT 457

Research Sponsored by the American Association of State Highway and Transportation Officials in Cooperation with the Federal Highway Administration

SUBJECT AREAS

Highway Operations, Capacity, and Traffic Control

Engineering Study Guide for Evaluating Intersection Improvements

JAMES A. BONNESON, P.E. AND

MICHAEL D. FONTAINETexas Transportation Institute

Texas A&M University

College Station, TX

Published reports of the

NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM

are available from:

Transportation Research BoardNational Research Council2101 Constitution Avenue, N.W.Washington, D.C. 20418

and can be ordered through the Internet at:

http://www.national-academies.org/trb/bookstore

Printed in the United States of America

NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM

Systematic, well-designed research provides the most effectiveapproach to the solution of many problems facing highwayadministrators and engineers. Often, highway problems are of localinterest and can best be studied by highway departmentsindividually or in cooperation with their state universities andothers. However, the accelerating growth of highway transportationdevelops increasingly complex problems of wide interest tohighway authorities. These problems are best studied through acoordinated program of cooperative research.

In recognition of these needs, the highway administrators of theAmerican Association of State Highway and TransportationOfficials initiated in 1962 an objective national highway researchprogram employing modern scientific techniques. This program issupported on a continuing basis by funds from participatingmember states of the Association and it receives the full cooperationand support of the Federal Highway Administration, United StatesDepartment of Transportation.

The Transportation Research Board of the National ResearchCouncil was requested by the Association to administer the researchprogram because of the Board’s recognized objectivity andunderstanding of modern research practices. The Board is uniquelysuited for this purpose as it maintains an extensive committeestructure from which authorities on any highway transportationsubject may be drawn; it possesses avenues of communications andcooperation with federal, state and local governmental agencies,universities, and industry; its relationship to the National ResearchCouncil is an insurance of objectivity; it maintains a full-timeresearch correlation staff of specialists in highway transportationmatters to bring the findings of research directly to those who are ina position to use them.

The program is developed on the basis of research needsidentified by chief administrators of the highway and transportationdepartments and by committees of AASHTO. Each year, specificareas of research needs to be included in the program are proposedto the National Research Council and the Board by the AmericanAssociation of State Highway and Transportation Officials.Research projects to fulfill these needs are defined by the Board, andqualified research agencies are selected from those that havesubmitted proposals. Administration and surveillance of researchcontracts are the responsibilities of the National Research Counciland the Transportation Research Board.

The needs for highway research are many, and the NationalCooperative Highway Research Program can make significantcontributions to the solution of highway transportation problems ofmutual concern to many responsible groups. The program,however, is intended to complement rather than to substitute for orduplicate other highway research programs.

Note: The Transportation Research Board, the National Research Council,the Federal Highway Administration, the American Association of StateHighway and Transportation Officials, and the individual states participating inthe National Cooperative Highway Research Program do not endorse productsor manufacturers. Trade or manufacturers’ names appear herein solelybecause they are considered essential to the object of this report.

NCHRP REPORT 457

Project G3-58 FY’99

ISSN 0077-5614

ISBN 0-309-06705-7

Library of Congress Control Number 2001-131981

© 2001 Transportation Research Board

Price $40.00

NOTICE

The project that is the subject of this report was a part of the National Cooperative

Highway Research Program conducted by the Transportation Research Board with the

approval of the Governing Board of the National Research Council. Such approval

reflects the Governing Board’s judgment that the program concerned is of national

importance and appropriate with respect to both the purposes and resources of the

National Research Council.

The members of the technical committee selected to monitor this project and to review

this report were chosen for recognized scholarly competence and with due

consideration for the balance of disciplines appropriate to the project. The opinions and

conclusions expressed or implied are those of the research agency that performed the

research, and, while they have been accepted as appropriate by the technical committee,

they are not necessarily those of the Transportation Research Board, the National

Research Council, the American Association of State Highway and Transportation

Officials, or the Federal Highway Administration, U.S. Department of Transportation.

Each report is reviewed and accepted for publication by the technical committee

according to procedures established and monitored by the Transportation Research

Board Executive Committee and the Governing Board of the National Research

Council.

http://www.national-academies.org/trb/bookstore/

FOREWORDBy Staff

Transportation ResearchBoard

This guide describes the engineering study process for evaluating the operationaleffectiveness of various intersection improvements. It also shows how capacity analy-sis and traffic simulation models can be used to assess the operational impacts of thoseimprovements. Use of this guide, particularly by junior traffic engineers, shouldenhance the decision-making process and reduce inappropriate installations of trafficcontrol signals. An enhanced version of this report is available on the world wide webat http://trb.org/trb/publications/nchrp/esg.pdf.

The Manual on Uniform Traffic Control Devices-Millennium Edition (MUTCD2000) states as a standard that an “engineering study of traffic conditions, pedestriancharacteristics, and physical characteristics of the location shall be performed to deter-mine whether installation of a traffic control signal is justified at a particular location.”The MUTCD 2000 furthers states as guidance that “a traffic control signal should notbe installed unless an engineering study indicates that installing a traffic control signalwill improve the overall safety and/or operation of the intersection.” The MUTCD 2000describes some aspects of the engineering study, including the traffic signal warrants,but does not attempt to fully describe the decision-making process.

Capacity analysis (e.g., the methods in the TRB Highway Capacity Manual) and traf-fic simulation models can be beneficially used in these engineering studies to assess theoperational impact of a traffic control signal and other intersection improvements. Some-times these tools may show that, while the warrants are met at a particular location, a lesscostly improvement would operate more effectively than a traffic control signal.

Under NCHRP Project 3-58, the Texas Transportation Institute analyzed difficultiescommonly faced when using traffic signal warrants to determine the appropriatenessof a traffic control signal. They also identified operational measures of effectivenessthat should be considered in the assessment of intersection improvements. They thendeveloped a guide to conducting an engineering study of an intersection.

Evaluating Intersection Improvements: An Engineering Study Guide defines thesteps involved in an engineering study of a problem intersection, beginning with iden-tifying the problem and viable alternative improvements to address the problem. It alsoillustrates how to use capacity analysis and traffic simulation models to determine themost effective operational improvement. The guide does not assist in the analysis ofthe safety or other impacts of the alternative improvements, although these must beconsidered when determining the most appropriate improvement. References to othersources of information on these types of analysis are provided.

The reader may be interested in an enhanced version of the guide available on theweb. That version includes internal hyperlinks between different parts of the report andexternal links to source material. This web version also includes 17 interactive work-sheets that can be helpful in using the guide.

1 CHAPTER 1 IntroductionOverview of the Assessment Process, 1

Alternative Identification and Screening, 1Engineering Study, 1Alternative Selection, 2

Objective and Scope, 2Objective, 2Scope, 2

Overview of the Guide, 2Other Impacts, 4

5 CHAPTER 2 Alternative Identification and ScreeningProcess, 5

Overview, 5Step 1. Define Problem and Cause, 5

1-a. Gather Information, 51-b. Define Problem and Cause, 7

Step 2. Select Candidate Alternatives, 82-a. Identify Potential Alternatives, 82-b. Organize and Select Alternatives, 8

Step 3. Select Viable Alternatives, 103-a. Gather Information, 113-b. Assess and Select Alternatives, 12

Guidelines, 13Guidelines for Use of Selected Geometric and Traffic Control Alternatives, 13

Overview, 13Add Flash Mode to Signal Control, 13Convert to Traffic Signal Control, 14Convert to Multi-Way Stop Control, 15Convert to Two-Way Stop or Yield Control, 16Prohibit On-Street Parking, 18Prohibit Left-Turn Movements, 19Convert to Roundabout, 19Add a Second Lane on the Minor Road, 20Add a Left-Turn Bay on the Major Road, 21Add a Right-Turn Bay on the Major Road, 22Increase Length of Turn Bay, 23Increase the Right-Turn Radius, 25

Conditions Affecting the Accuracy of Conclusions from the Signal Warrant Check, 26

Overview, 26Right-Turn Volume on the Minor Road, 26Heavy Vehicles on the Minor Road, 27Pedestrian Volume, 27Progressive Traffic Flow on the Major Road, 28Three-Leg Intersection, 28Added Through Lane on the Major Road, 29Left-Turn Bay, 29Right-Turn Bay on the Minor Road, 29Wide Median on the Major Road, 30Combination of Factors, 30

31 CHAPTER 3 Engineering StudyProcess, 31

Overview, 31Step 1. Determine Study Type, 31

1-a. Determine Type of Operation, 311-b. Determine Type of Evaluation, 32

Step 2. Select Analysis Tool, 332-a. Identify Desired Capabilities, 332-b. Evaluate and Select Analysis Tool, 33

CONTENTS

Step 3. Conduct Evaluation, 363-a. Gather Information, 363-b. Evaluate Operational Performance, 383-c. Determine Alternative Effectiveness, 38

Guidelines, 39Guidelines for Designing the Signalized Intersection Alternative, 39

Overview, 39Intersection Geometry, 40Controller Operation, 41Controller Phase Sequence, 42Basic Controller Settings, 44Controller Settings for Isolated Operation, 45Controller Settings for Coordinated Operation, 52

Guidelines for Use of Stochastic Simulation Models, 56Overview, 56Random Arrivals, 56Random Number Seed, 56Minimum Simulation Initialization Time, 56Simulation Period and Run Duration, 56

60 CHAPTER 4 Alternative SelectionProcess, 60

Overview, 60Step 1. Identify Impacts, 60

1-a. Identify Decision Factors, 601-b. Assess Degree of Impact, 61

Step 2. Select Best Alternative, 61Step 3. Document Study, 63

64 BIBLIOGRAPHY

65 REFERENCES

A-1 APPENDIX A Case Study Applications

B-1 APPENDIX B MUTCD Traffic Control Signal Warrant Worksheets

C-1 APPENDIX C Data for Guideline Evaluation

AUTHOR ACKNOWLEDGMENTSThe research team would like to recognize the support and guid-

ance provided by the project panel. The project panel membersinclude Dr. Robert Wortman (chair), University of Arizona; Mr.Rick Bennett, Missouri Department of Transportation; Dr. ThomasH. Culpepper, Wilbur Smith and Associates; Ms. Beth P. DeAngelo, Parsons Brinkerhoff—FG, Inc.; Mr. Glenn M. Grigg, con-sultant; Mr. Para M. Jayasinghe, City of Boston, Massachusetts;Mr. Steve G. Jewell, City of Columbus, Ohio; Mr. Ken Kobetsky,AASHTO; Mr. Pawan Maini, Fitzgerald and Halliday, Inc.; and Mr.Henry Lieu, Federal Highway Administration.

Finally, the research team would like to acknowledge the engi-neers who participated in the focus group assembled for the researchproject. These individuals include Mr. W. Martin Bretherton, Jr.,Gwinnett County Department of Transportation, Georgia; Mr. RonNorthouse, City of San Jose, California; Mr. Michael R. Scanlon,City of Tampa, Florida; Mr. Richard Filkins, Washington StateDepartment of Transportation; and Mr. Chad J. Smith, Iowa Depart-ment of Transportation. Their thoughtful advice and comment onthe procedures and guidelines are greatly appreciated.

Abbreviations used without definitions in TRB publications:

AASHO American Association of State Highway OfficialsAASHTO American Association of State Highway and Transportation OfficialsASCE American Society of Civil EngineersASME American Society of Mechanical EngineersASTM American Society for Testing and MaterialsFAA Federal Aviation AdministrationFHWA Federal Highway AdministrationFRA Federal Railroad AdministrationFTA Federal Transit AdministrationIEEE Institute of Electrical and Electronics EngineersITE Institute of Transportation EngineersNCHRP National Cooperative Highway Research ProgramNCTRP National Cooperative Transit Research and Development ProgramNHTSA National Highway Traffic Safety AdministrationSAE Society of Automotive EngineersTCRP Transit Cooperative Research ProgramTRB Transportation Research BoardU.S.DOT United States Department of Transportation

Advisers to the Nation on Science, Engineering, and Medicine

National Academy of SciencesNational Academy of EngineeringInstitute of MedicineNational Research Council

The Transportation Research Board is a unit of the National Research Council, which serves the National Academy of Sciences and the National Academy of Engineering. The Board’s mission is to promote innovation and progress in transportation by stimulating and conducting research, facilitating the dissemination of information, and encouraging the implementation of research results. The Board’s varied activities annually draw on approximately 4,000 engineers, scientists, and other transportation researchers and practitioners from the public and private sectors and academia, all of whom contribute their expertise in the public interest. The program is supported by state transportation departments, federal agencies including the component administrations of the U.S. Department of Transportation, and other organizations and individuals interested in the development of transportation.

The National Academy of Sciences is a private, nonprofit, self-perpetuating society of distin-guished scholars engaged in scientific and engineering research, dedicated to the furtherance of science and technology and to their use for the general welfare. Upon the authority of the charter granted to it by the Congress in 1863, the Academy has a mandate that requires it to advise the federal government on scientific and technical matters. Dr. Bruce M. Alberts is president of the National Academy of Sciences.

The National Academy of Engineering was established in 1964, under the charter of the National Academy of Sciences, as a parallel organization of outstanding engineers. It is autonomous in its administration and in the selection of its members, sharing with the National Academy of Sciences the responsibility for advising the federal government. The National Academy of Engineering also sponsors engineering programs aimed at meeting national needs, encourages education and research, and recognizes the superior achievements of engineers. Dr. William A. Wulf is president of the National Academy of Engineering.

The Institute of Medicine was established in 1970 by the National Academy of Sciences to secure the services of eminent members of appropriate professions in the examination of policy matters pertaining to the health of the public. The Institute acts under the responsibility given to the National Academy of Sciences by its congressional charter to be an adviser to the federal government and, upon its own initiative, to identify issues of medical care, research, and education. Dr. Kenneth I. Shine is president of the Institute of Medicine.

The National Research Council was organized by the National Academy of Sciences in 1916 to associate the broad community of science and technology with the Academy’s purpose of furthering knowledge and advising the federal government. Functioning in accordance with general policies determined by the Academy, the Council has become the principal operating agency of both the National Academy of Sciences and the National Academy of Engineering in providing services to the government, the public, and the scientific and engineering communities. The Council is administered jointly by both the Academies and the Institute of Medicine. Dr. Bruce M. Alberts and Dr. William A. Wulf are chairman and vice chairman, respectively, of the National Research Council.

1

CHAPTER 1

INTRODUCTION

The Manual on Uniform Traffic Control Devices-Millennium Edition (MUTCD 2000) (1) includes warrantsthat set minimum thresholds for considering the installationof a traffic signal. The threshold values used in these war-rants were established many years ago. They are based largelyon engineering judgment and reflect sensitivity to two vari-ables: approach volume and number of lanes. However,their subjective basis and limited sensitivity can make theminaccurate predictors of the need for signal control at someintersections and could result in the unnecessary installa-tion of traffic signals. In fact, two separate evaluations ofthe MUTCD warrants found that they do not always yieldconclusions that agree with engineering judgment (2,3).This finding is a concern because a recent literature reviewby Bonneson and Fontaine (4) revealed that delays and stopscan increase 100 to 200 percent when marginally warrantedsignals are installed at an intersection.

To eliminate the unnecessary installation of traffic signals,the text of the MUTCD was revised for its 1988 publication.The purpose of the revisions was to indicate clearly that thedecision to install a traffic signal should be based on the find-ings from an engineering study. The 1998 MUTCD statedthat “satisfaction of a warrant or warrants is not in itself jus-tification for a signal.” This text was retained for the MUTCD2000 and further states that “A traffic control signal shouldnot be installed unless an engineering study indicates thatinstalling a traffic control signal will improve the overallsafety and/or operation of the intersection.” The intent of thisrecommendation is to encourage consideration of the fullrange of intersection improvement alternatives and selectionof the most effective alternative for implementation.

This guide documents the steps involved in the formalengineering study of improvement alternatives and focuseson the use of capacity analysis procedures and simulationmodels (i.e., analysis tools) to evaluate the operational im-pacts of improvement alternatives. Case studies illustrate howthese analysis tools can sometimes be used to show that anintersection would operate more effectively without a trafficsignal, even though a signal warrant is met.

OVERVIEW OF THE ASSESSMENT PROCESS

Generally, the engineering assessment process is initiatedwhen operational or safety problems are identified at an un-signalized intersection. The goal of this process is to identify

the most effective solution to the identified problem. Theassessment process is often integral to a larger, more com-prehensive system management process in which all prob-lematic transportation facilities are considered for improve-ment. The stages of the system management process areshown in Figure 1-1.

The assessment process represents three specific stages ofthe system management process. In the first stage, viable al-ternatives are identified. In the second stage, an engineeringstudy is conducted to evaluate the effectiveness of each viablealternative. Finally, the best alternative is selected on the basisof its effectiveness and its other, non-motorist-related effects.

Following the assessment process, the best alternative fora given intersection is pooled with other improvement proj-ects for funding consideration. If funding is available, thealternative is constructed, and the intersection is monitoredto confirm that the original problem has been solved.

Alternative Identification and Screening

During the alternative identification and screening stage,the traffic engineer makes a preliminary assessment of theproblem and identifies viable intersection improvement alter-natives. Alternatives may include changes in traffic control,intersection geometry, or both. Traffic control alternativescan include two-way stop control, multi-way stop control, or signal control. Geometric alternatives that are sometimesidentified include the addition of left or right-turn bays on themajor or minor roadways.

Initially, alternatives are identified that could solve theobserved problem. Then, this list of candidate alternatives isreduced, using formal guidelines or warrants, to include onlythe most viable alternatives. Formally recognized guidelinesexist for traffic signals, multi-way stop control, and somemajor geometric improvements. Guidelines for control deviceapplications are provided in the MUTCD 2000 (1). Similarly,guidelines that describe when a left-turn bay is needed on themajor-road approach to an unsignalized intersection are pro-vided in A Policy on Geometric Design of Highways andStreets (Green Book) (5).

Engineering Study

During the engineering study stage, the viable alternativesare evaluated in terms of their effect on road users and the

immediate environment. The accurate assessment of theseeffects typically requires the application of formally recog-nized analysis procedures. However, in some less-complexsituations, experience with similar alternatives and operatingconditions may be sufficient to enable the engineer to estimatean alternative’s effectiveness.

Alternative Selection

During the alternative selection stage, the engineer assessesthe effects of each alternative and then selects the “best” onefor implementation. Effects considered may include improve-ments in traffic operations or safety and disruption to area aes-thetics, the environment, or adjacent property. Selection ofthe best alternative may also reflect consideration of the con-struction cost of each alternative. The method of selection canvary from a complicated life-cycle, benefit-cost analysis to asimple identification of the alternative that yields the leasttraffic delay. The methods used and the effects consideredwill depend on the conditions present at the problem location.

OBJECTIVE AND SCOPE

Objective

This guide has two objectives: (1) to define the steps in-volved in an engineering study of a problem intersection and(2) to provide guidelines for using capacity analysis or simu-lation to determine the most effective alternative on the basisof operational considerations. To achieve these objectives,the coverage of the guide is expanded to include the threestages of the assessment process, as shown in Figure 1-1. Thisapproach provides a comprehensive treatment of problemintersections by preceding the engineering study stage with

2

an alternative identification and screening stage and follow-ing it with an alternative selection stage. Application of allthree stages will ensure that a wide range of alternatives isconsidered and that the alternative selected is effective.

Scope

The guide is intended to describe how to evaluate the oper-ational effects of alternative geometrics and control modes ata problem intersection. This description includes guidelineson (1) the alternative selection process and (2) the use ofcapacity analysis procedures and simulation models for alter-native evaluation. The assessment of an alternative’s safety(or other) effect is beyond the scope of the guide. The analystis encouraged to consult the References and Bibliography forguidance on safety evaluations.

The target audience of this guide is the junior traffic engi-neer. Procedures and guidelines provided herein are intendedto identify critical decision points and problem-solving tech-niques for engineers with only a few years of experience. Tomeet the needs of this intended audience, each topic is coveredthoroughly. Sometimes, the material herein may contradict thepolicies and procedures of the reader’s agency. In such in-stances, agencies may substitute their policies or practices forthose in this guide.

This guide is intended to be applicable to intersectionswith a wide range of control modes. However, the three modescommonly found at U.S. intersections are given greater em-phasis in some sections of the document. These modes are asfollows:

1. Two-way stop control,2. Multi-way stop control, and3. Signal control.

Although the three control modes listed above are empha-sized, the procedures in the guide can be used to evaluateother control modes (e.g., no control, two-way yield control,and roundabouts).

OVERVIEW OF THE GUIDE

This guide consists of four chapters and three appendixes.Chapters 2, 3, and 4 describe the three stages of the assess-ment process, as shown in Figure 1-1. A flowchart diagram-ming the assessment process is shown in Figure 1-2.

Chapter 2 of the guide describes the alternative identifica-tion and screening stage. The chapter describes how to iden-tify the problems at the subject intersection, how to identifycandidate improvement alternatives, and how to screen thesealternatives so that only the most viable alternatives are eval-uated in the engineering study.

Chapter 3 of the guide describes the engineering studystage. This chapter describes the process for evaluating the

Figure 1-1. Stages of the system management process.

3

Figure 1-2. Flow chart of the assessment process.

effectiveness of the selected alternatives. The process focuseson how to evaluate traffic operations by using capacity analy-sis procedures or simulation models. Guidelines are providedto help the analyst determine the most appropriate analysistool (i.e., capacity analysis procedure or simulation model),the data needed for the evaluation, and whether or not analternative operates satisfactorily.

Chapter 4 of the guide describes the alternative selectionstage, including how to identify the best alternative for agiven intersection. Alternative selection is described in gen-eral, rather than precise, terms, so as to allow the analystsome flexibility in selecting the types of effects to considerwhen selecting an alternative and the relative weight to beapplied to each effect.

Guidelines that support the assessment process are pro-vided in the Guidelines section of Chapters 2 and 3. Thematerials provided include (1) a list of traffic control andgeometric design alternatives, (2) warrants and guidelines de-scribing conditions suitable for selected alternatives, (3) tech-niques for designing the traffic signal control alternative, and(4) guidelines for using stochastic simulation models. Use ofthis guide should help agencies to maximize their return oninfrastructure investment and that they will avoid the unnec-essary installation of traffic signals.

The appendixes to the guide are intended to provide sup-plementary information that makes the guide a more versatiledocument. The use of the guide is demonstrated in severalcase study situations in Appendix A. Appendix B provides

4

worksheets for documenting the MUTCD signal warrant check.Appendix C describes techniques that can be used to gatheror derive the data needed for the engineering study.

Although the coverage in the guide is thorough, it is as-sumed that the reader will have access to two reference doc-uments: (1) the current edition of the MUTCD and (2) theManual of Transportation Engineering Studies (6). This lat-ter document describes procedures for collecting data for thedirect evaluation of intersection performance and for collect-ing the input data needed for a capacity analysis procedure orsimulation model.

Other Impacts

Although the focus of the guide is on evaluating the oper-ational effects of improvement alternatives, the analyst shouldalso consider safety and other effects during the engineeringstudy. Safety impact assessment may range from an informalsubjective assessment to a formal quantitative evaluation,depending on the types of problems being experienced (oranticipated) at the subject intersection. Often, the best alter-native will be the one that improves traffic operation andenhances safety. However, this may not always be the case—some alternatives may improve operation but degrade safetyand vice versa. Therefore, the analyst should carefully eval-uate and weigh all relevant effects when selecting the bestalternative.

5

CHAPTER 2

ALTERNATIVE IDENTIFICATION AND SCREENING

Intersections that are inefficient or unsafe are typicallysubjected to some type of engineering assessment to identifythe underlying problem and its solution. The engineeringassessment process consists of three stages: (1) alternativeidentification and screening, (2) the engineering study, and(3) alternative selection. The first stage of the assessmentprocess is the subject of this chapter. The steps involved inthe alternative identification and screening stage are definedin the first section of this chapter. Guidelines are provided inthe second section. These guidelines describe (1) conditionswhere selected improvement alternatives may be helpful and(2) conditions that might mislead the signal warrant check.

PROCESS

Overview

The alternative identification and screening stage consistsof three steps. These steps are as follows:

1. Define problem and cause,2. Select candidate alternatives, and3. Select viable alternatives.

In the first step, the problem is defined and its cause is identi-fied through the conduct of a site visit and the collection of rel-evant existing data. Then, several alternatives are selected forfurther consideration. Finally, these alternatives are screenedusing available engineering guidelines so that a subset list ofviable alternatives is identified.

The objective of the alternative identification and screen-ing stage is to determine if a problem exists and, if it does, toidentify one or more viable alternatives that can eliminate or mitigate the problem. This objective is achieved throughan evaluation of existing conditions and consideration of arange of improvement alternatives. The steps in this processare described in the remainder of this section.

Step 1. Define Problem and Cause

The first step in the alternative identification and screen-ing stage is to determine the nature of the problem at an inter-section and to define its potential causes. The tasks involvedin making this determination are to

a. Gather information andb. Define the problem and cause.

In the first task, data are collected that describe the inter-section’s history and its present condition in terms of trafficvolume, safety record, and geometric layout. Then, these dataare used to define the intersection’s operational or safetyproblems and to identify their causes.

The assessment process typically begins when the engi-neer is notified of a problem at an intersection or series ofintersections. This notification can come from sources thatare either internal or external to the agency. Some possiblesources of problem reports include

• Complaints from local residents about existing inter-sections,

• Developers seeking traffic control for proposed inter-sections,

• Observations from field crews,• Data obtained from consultant studies,• Information obtained from regional traffic counts,• Potential problematic conditions identified through

regional traffic projections, and• Annual safety analysis of high-crash locations.

1-a. Gather Information

Overview. Following notification of a problem, the engi-neer makes a preliminary assessment to determine the extentof the problem and whether it requires a solution. This as-sessment involves gathering readily available informationabout the problem intersection so that the problem can bedefined and its cause identified. Information sources includehistoric file data, firsthand observation, and a site survey.

Completion of this task should not require a rigorous datacollection effort. A visit to the site should provide sufficientinformation to determine if a problem exists. Traffic volume,conflict, or delay data should not be needed to make this deter-mination. In Step 3 of the process, Select Viable Alternatives,additional data will be collected to allow a more detailedassessment of the problem and its solution.

Historic Data. The analyst should gather existing engi-neering studies, corridor studies, traffic studies, and crash

data summaries that may help to identify potential problemsat the subject intersection. Crash data summarized in a colli-sion diagram may indicate where problems are occurring atthe intersection. (Details on the construction of a collisiondiagram are provided in Appendix C.)

Observational Study. An important component of theproblem-cause identification process is the firsthand obser-vation of traffic operations. A visit to observe operationsshould be scheduled to coincide with the occurrence of thereported problems (e.g., p.m. peak traffic demand hour). Ini-

6

tially, the engineer should drive through the subject intersec-tion and attempt to experience the problem. Then, the engi-neer should observe intersection operation from a curbsidevantage point. An onsite observation report, such as thatshown in Figure 2-1, should be completed during the site visit.(Details regarding the completion of this report and a blankreport form are provided in Appendix C.)

Site Survey. The engineer should also have a survey ofsite conditions conducted. The survey data should be recordedon a condition diagram. The diagram represents a plan-view,

Figure 2-1. Sample onsite observation report.

scale drawing of the subject intersection. Conditions recordedmay include road width, pavement markings, speed limits,and traffic control devices. (Details of the site survey and ablank condition diagram are provided in Appendix C.)

Traffic Projections. Traffic volume projections are im-portant when a new intersection is being proposed. Projectedturning movement counts may provide the best data avail-able for the analyst to determine the most appropriate type ofcontrol for the intersection. The analyst should also deter-mine if major changes in traffic patterns or land use are antic-ipated in the vicinity of the subject intersection. Improvementalternatives should be able to accommodate future trafficpatterns.

1-b. Define Problem and Cause

Overview. During this task, the analyst will define theproblem and identify its cause. The information gathered inthe preceding task is used for this purpose. Initially, the ana-lyst will review this information and determine if sufficientevidence of a problem exists. If it is determined that a prob-lem exists, then the problem is formally defined and its causeis identified.

7

Assess Evidence. After reviewing the available historicalinformation and information obtained from the site visit, theengineer should determine if sufficient evidence of a prob-lem exists to proceed further in the assessment process. Ifsuch evidence exists, then further study may be necessary.

Define Problem and Identify Cause. Problems can gen-erally be categorized as operational or safety-related. Opera-tional problems are typically associated with excessive delayto one or more traffic movements. Safety-related problemsare typically associated with frequent conflicts, erratic maneu-vers, non-compliance with control devices, and collisions.The information obtained during Task 1-a should be used todetermine which (if any) of these two categories of problemsexist for each of the intersection traffic movements.

Once the problem has been defined, the engineer shouldidentify what is causing the problem. Problems and potentialcauses are listed in Table 2-1. The possible causes for eachproblem category (i.e., delay or conflict) are identified bycheckmark (✓ ). When using Table 2-1, each intersection ap-proach should be individually evaluated. The movementsexperiencing a problem should be identified and comparedwith the checked combinations shown. The terms “delay”and “conflict” refer broadly to operational and safety prob-lems observed during the site visit (they are not meant to

TABLE 2-1 Common operational problems and possible causes

imply that a delay or conflict study must be conducted beforeTable 2-1 can be consulted).

To illustrate the use of Table 2-1, consider a minor-roadapproach at a four-leg intersection. All three movements onthis approach were observed to have excessive delay (but noconflicts). Table 2-1 indicates that three possible causes areassociated with the observed minor-road delays: (1) inade-quate capacity for minor-road movements, (2) inadequate sep-aration of minor-road movements, and (3) staggered arrival ofmajor-road platoons. These possible causes are advanced tothe next step to determine candidate improvement alternatives.

Define Influence Area. Designation of the intersection“influence area” is an important part of this task. This area mayinclude the subject intersection only or it may include the net-work of roads that surround the subject intersection. As a min-imum, the influence area should extend from the subject inter-section sufficiently far so as to include the queues associatedwith the intersection’s traffic movements.

The extent of the influence area’s coverage is based pri-marily on the proximity of the subject intersection to otherintersections. The inclusion of nearby intersections in theinfluence area would be based in part on the answers to Ques-tions A, B, C, and D on the observation report (Figure 2-1).If one or more of these questions are answered “Yes” or “NotSure,” then the subject intersection may be influenced by oneor more nearby intersections.

Efforts to improve conditions at the subject intersectionshould be sensitive to the operation of all intersections in theinfluence area. All intersections in the influence area shouldbe included in the evaluation conducted in the engineeringstudy stage (see Chapter 3).

Step 2. Select Candidate Alternatives

After the problem is defined and its likely cause is identified,one or more candidate alternatives should be selected. Thissection provides a procedure for selecting candidate improve-ment alternatives. The procedure consists of the followingtwo tasks:

a. Identify potential alternatives andb. Organize and select alternatives

During the first task, a list of alternatives that could solve theproblems is identified. Then, these alternatives are organizedand screened to obtain a subset of candidate alternatives basedon site constraints and institutional preferences.

2-a. Identify Potential Alternatives

Overview. Tables 2-2 and 2-3 are provided in this sec-tion to assist in the identification of potential improvementalternatives. The first table applies to problems that require

8

intersection-specific solutions and the second table appliesto intersections requiring system-related solutions. The alter-natives listed in these tables emphasize operational improve-ments; the analyst is referred to Chapter 11 of the Manual ofTransportation Engineering Studies (6 ) for a complete list ofsafety problems, causes, and possible solution alternatives.

Intersection-Specific Alternatives. Table 2-2 identifiesproblems commonly encountered at intersections. This tablealso lists potential corrective strategies and candidate alter-natives. Problems and causes other than those listed may alsobe known to exist at a specific intersection. In these situa-tions, judgment should be used to identify the appropriateimprovement alternatives.

The characteristics of the potential alternatives listed inTable 2-2 are indicated by underline and italic font. Alter-natives that affect traffic operations (e.g., motorist delay)are underlined. Alternatives shown in italics should be eval-uated for viability in the next step (i.e., Step 3 - SelectViable Alternatives).

To illustrate the use of Table 2-2, consider a minor-roadintersection approach observed to have excessive delay causedby inadequate separation of its traffic movements. Table 2-2indicates that one corrective strategy is to separate the con-flicting movements by using one of the following alternatives:(1) add a second lane on the approach or (2) increase right-turnradius. This latter alternative would widen the throat of theapproach and effectively separate right-turning vehicles fromthrough and left-turning vehicles.

System-Related Alternatives. Table 2-3 lists the prob-lems that may be found at non-isolated intersections. Theseintersections have an influence area that extends beyond thelimits of the subject intersection and includes the adjacentintersections (either unsignalized or signalized). The improve-ment alternatives listed in both Table 2-2 and Table 2-3 shouldbe considered when the intersection is not isolated. The alter-natives that affect traffic operations are underlined; thosealternatives whose viability can be evaluated in the next stepare italicized.

2-b. Organize and Select Alternatives

Overview. During this task, the potential alternatives arescreened to determine if they are suitable candidates for thesubject location and whether their immediate implementa-tion is more cost-effective than the conduct of a formal engi-neering study. If no alternatives are selected for immediateimplementation, the alternatives that remain after screeningrepresent the “candidate” improvement alternatives. Thesealternatives are then advanced to Step 3.

Identify Candidate Alternatives. Initially, the list ofpotential improvement alternatives should be screened for

9

TABLE 2-2 Potential intersection-specific, engineering improvement alternatives

suitability on the basis of firsthand knowledge of site con-straints and agency preferences. For example, if the site beingstudied is in a downtown area where right-of-way is limited,the analyst may choose to eliminate options that will requirethe acquisition of significant amounts of right-of-way. Thisscreening is performed without the collection of additionaldata, so only options that would clearly not be acceptableshould be eliminated.

Identify Alternatives Suitable for Immediate Imple-mentation. All low-cost alternatives should be considered forimmediate implementation and evaluation. An alternative’s“cost” includes the direct cost of its implementation as well asany indirect cost to adjacent land users and the environment.Low-cost alternatives are defined as those alternatives thathave a cost that is significantly less than that of the formalengineering study. Typical low-cost alternatives include ad-visory signing, revised pavement markings, and vegetationremoval (to improve sight lines).

If any low-cost alternatives have been identified, the engi-neer can proceed directly to the implementation stage. In thisstage, the low-cost alternative(s) would be programmed for

10

implementation and monitored for effectiveness. If a follow-up observational study reveals that the alternatives imple-mented in this manner have not substantially improved thereported problems, then a new assessment process should be initiated.

Step 3. Select Viable Alternatives

The third step in the alternative identification and screen-ing stage is to determine the viability of the candidatealternatives identified in Step 2. The tasks in making thisdetermination are to

a. Gather information andb. Assess and select alternatives

In the first task, relevant guidelines are identified, and corre-sponding data are collected to facilitate the evaluation of thecandidate alternatives identified in Step 2. Then, in the sec-ond task, the guidelines are used to determine which of thecandidate alternatives are more viable than the others.

TABLE 2-3 Potential system-related, engineering improvement alternatives

The purpose of Step 3 is to screen out those alternativesthat are not likely to have a significant positive effect on inter-section operations. By screening out these alternatives, it ishoped that the list of alternatives to be evaluated during theengineering study (described in Chapter 3) will be reduced sothat it includes only the most promising alternatives.

3-a. Gather Information

Overview. The procedure for selecting viable alternativesis based on a review of guidelines that indicate when animprovement alternative is likely to be effective. These guide-lines are provided in the Guidelines section of this chapter. Ifdesired, other guidelines may be added or substituted by theresponsible agency. The Guidelines section addresses thefollowing alternatives:

• Add flash mode to signal control,• Convert to traffic signal control,• Convert to multi-way stop control,• Convert to two-way stop control,• Convert to two-way yield control,• Prohibit on-street parking,• Prohibit left-turn movements,• Convert to roundabout,• Add a second lane on the minor road,• Add a left-turn bay on the major road,• Add a right-turn bay on the major road,• Increase the length of the turn bay, and• Increase right-turn radius.

(The order in which these alternatives are listed is arbitrary andis not intended to convey any sense of priority or importance.)

Alternative Categories. Some alternatives offered inTables 2-2 and 2-3 are not included in the list above. Theseomissions occurred because (1) there is no formal guidanceavailable in the literature, or (2) the alternative’s effect cannotbe quantified in terms of delay. Therefore, it is possible thatonly a subset of the candidate alternatives can be evaluated inthis step.

Table 2-4 indicates the action to be taken based on thealternative’s effect and guideline availability. At the onset ofthis task, Table 2-4 should be consulted for each candidate

11

alternative to determine the appropriate action. Alternativesin Category I should be assessed using the process describedin this section.

Identify Data. The next activity to be undertaken for thistask is to gather the data needed to apply the guidelines appli-cable to each Category-I alternative. The specific types of dataneeded to evaluate each guideline are listed in Table 2-5. Thelist is generally complete for all of the guidelines shown;however, additional data may be needed in specific situa-tions. In all cases, the analyst should review the guideline, asdescribed in the Guidelines section, to determine if additionaldata are needed. Data needed for one guideline, ProhibitLeft-Turn Movements, are not listed in the table because theyare unique; however, they are described in the Guidelinessection of this chapter.

Before gathering any data, the analyst should identify thespecific data needed for the collective set of guidelines tobe evaluated. Frequently, data needed for one guideline canbe used for another guideline. The data collection plan shouldtake advantage of such overlap to avoid redundancy.

If the traffic signal or multi-way stop control alternative isbeing considered, the analyst should note that only one com-ponent of the respective warrants or criteria need to be satis-fied to designate the alternative as “viable.” Thus, the analystshould determine which of the signal warrants and the multi-way stop control criteria will be evaluated and collect dataonly for these selected warrants and criteria. Often, the prob-lems and causes identified in Step 2 will direct the selectionof the warrants or criteria to be evaluated. When this is notthe case, Hawkins and Carlson (7 ) recommend evaluation ofMUTCD 2000 Warrants 1, 2, and 3 as a first step, because theassociated data require the least effort to collect.

Collect Data. The procedures for collecting the data neededto evaluate the selected guidelines will vary depending onwhether the subject intersection exists or is proposed for con-struction. For existing intersections, appropriate data collec-tion procedures are described in the Manual of Transporta-tion Engineering Studies (6). For proposed intersections,techniques described in Appendix C can be used to estimateturn movement volumes from forecast average daily trafficdemands. Regardless of the source, the data should representtraffic conditions occurring on an “average day” (i.e., a day

TABLE 2-4 Categories of candidate alternatives

representing traffic volumes normally and repeatedly found ata location).

3-b. Assess and Select Alternatives

Overview. During this task, the Category-I alternatives areevaluated in order to develop a subset list of viable alterna-tives. The information gathered in Task 3-a is used to evalu-ate each alternative using the appropriate guideline. The stepsinvolved in the application of each guideline are listed in theGuidelines section. Those alternatives that satisfy their corre-sponding guideline should be considered “viable” and evalu-ated more formally during the engineering study (described inChapter 3).

At this point, some alternatives may appear less promisingthan others do in terms of their ability to solve a problem, andthe analyst may be tempted to drop the less promising alter-natives. However, these alternatives may provide the best bal-

12

ance between implementation impact and improvement ef-fectiveness. The most appropriate alternative can only beaccurately identified after the effectiveness of each alterna-tive has been evaluated during the engineering study. In short,all viable alternatives should be advanced to the engineeringstudy stage.

Traffic Signal Alternative Issues. Two issues should beaddressed when the traffic signal alternative is found to beviable (i.e., one or more warrants are satisfied). Considerationof these issues is important because of the potential for thesignal to affect intersection operation or safety negatively.The first issue relates to the belief of some engineers that thetraffic signal is the most direct means of solving all intersec-tion problems. The second issue relates to the presence ofatypical (or problematic) conditions that may reduce the util-ity of the warrant check.

With regard to the first issue, the analyst might be inclinedto drop all other alternatives when traffic signal control is

TABLE 2-5 Data needed to evaluate guidelines

found to be viable. However, the consequences of droppingthe other alternatives can be significant. For example, stud-ies show that when stop control is converted to signal controland the intersection volume “just” satisfies a warrant, theresulting overall delay often increases. Specifically, Williamsand Ardekani (8) found that delays increased by as much as 113 percent; Kay et al. (9) found that delays increased 200 percent; and Bissell and Neudorff (10) found that delaysincreased by 10 seconds per vehicle (s/veh) when traffic sig-nals were used to replace two-way stop control. Therefore,when the signal control alternative is found to be viable, theanalyst is strongly encouraged to identify and advance otheralternatives to the engineering study stage.

Regarding the second issue, atypical or unusual traffic,signalization, or geometric conditions may reduce the accu-racy of the results of the warrant check. Some of the morefrequently encountered problematic conditions include thefollowing:

• Right-turn volume on the minor road,• Heavy vehicles on the minor road,• Pedestrian volumes,• Progressive traffic flow on the major road,• Three-leg intersection,• Added through lane on the minor road,• Left-turn bay,• Right-turn bay, and• Wide median on the major road.

(The order in which the conditions are listed is arbitrary andis not intended to convey any sense of priority or importance.)

With regard to the second issue, guidance provided in theGuidelines section can be used to determine if a problematiccondition is likely to affect the results of the warrant check. Ifa problematic condition exists, then the effect of the conditionon intersection operations should be carefully evaluated in theengineering study stage.

GUIDELINES

This section provides guidelines that can be used during thealternative identification and screening stage. These guide-lines are presented in two separate sections with the followingtitles:

1. Guidelines for Use of Selected Geometric and TrafficControl Alternatives.

2. Conditions Affecting the Accuracy of Conclusions fromthe Signal Warrant Check.

The first section describes guidelines that can be used toevaluate the viability of 13 alternatives. The second sectiondescribes nine problematic traffic, signalization, or geomet-ric conditions that reduce the accuracy of the results of thewarrant check.

13

Guidelines for Use of Selected Geometric and Traffic Control Alternatives

Overview

This section provides guidelines that can be used to deter-mine when various traffic control devices and geometric ele-ments may be helpful in improving intersection operations orsafety. These guidelines were obtained primarily from refer-ence documents that are generally recognized as authoritativeguides on engineering practice. Other sources for the guide-lines include reports documenting significant research effortswhose recommendations are intended for nationwide appli-cation. The alternatives for which guidelines are provided inthis section include

• Add flash mode to signal control,• Convert to traffic signal control,• Convert to multi-way stop control,• Convert to two-way stop or yield control,• Prohibit on-street parking,• Prohibit left-turn movements,• Convert to roundabout,• Add a second lane on the minor road,• Add a left-turn bay on the major road,• Add a right-turn bay on the major road,• Increase the length of the turn bay, and• Increase the right-turn radius.

(The order in which the alternatives are listed is arbitrary andis not intended to convey any sense of priority or importance.)

The guidelines described in this section tend to be conser-vative such that they indicate only when an alternative maybe helpful (i.e., viable). If an alternative is found to satisfy theguideline threshold conditions, then the effectiveness of thealternative should be verified through the conduct of an engi-neering study (as described in Chapter 3). The results of theengineering study should form the basis for any recommen-dation to implement an alternative. In contrast, if the guide-line is not satisfied, then it should be assumed that the corre-sponding alternative is not viable and should be droppedfrom further consideration.

Finally, the guidelines provided in this section do not in-clude all possible improvement alternatives. The guidelinesprovided are believed to correspond to the more commonlyimplemented alternatives. Judgment may be needed to iden-tify viable alternatives for intersections that have unique oper-ating or geometric conditions or when several alternatives areproposed for use in combination at a specific intersection.

Add Flash Mode to Signal Control

Introduction. Intersections with light-to-moderate trafficdemands may benefit from traffic signal control during the

hours of peak demand but may not derive benefit during off-peak hours. When this benefit results from a reduction inmotorist delay, it may be useful to operate the signal in a flashmode during the off-peak hours. Flash mode may consist ofa yellow/red combination or a red/red combination. For theyellow/red combination, the major road is flashed yellow andthe minor road is flashed red.

Pusey and Butzer (11) indicate that flash mode operationhas several benefits. These benefits can include

• Reduced stops and delays to major-road traffic,• Reduced delay to minor-road traffic,• Reduced electrical consumption, and• Reduced vehicular fuel consumption and traffic noise.

A review of the literature by Kacir et al. (12) revealed thatyellow/red flash mode may be associated with higher crashrates, especially when the ratio of major-road-to-minor-roadvolume is less than 2.0.

Guidance. Benioff et al. (13) define conditions in whichflash mode is not likely to cause safety problems. Specifi-cally, they recommend using flashing yellow/red when (1) thetotal major-road volume is less than 200 vehicles per hour(veh/h), or (2) when the ratio of major-road-to-minor-roadvolume is greater than 3.0. This recommendation is illustratedin Figure 2-2 as the unshaded area.

Kacir et al. (12) compared the delays produced by flashingyellow/red with those produced by traffic signal control. Theyfound that delays were significantly reduced when (1) themajor-road-to-minor-road volume ratio is greater than 3.0, (2) the major-road volume is less than 250 vehicles per hourper lane (veh/h/ln), and (3) the higher approach minor-roadvolume is less than 85 veh/h/ln. These recommendationswere incorporated in Figure 2-2 with dashed lines (they are

14

based on an assumed directional distribution of 55/45 percentfor both roadways).

Application. The guidelines stated in the preceding sec-tion (and shown in Figure 2-2) are intended to minimize theoperational impact of a traffic signal and maintain a reason-able degree of safety. This guideline assumes that traffic sig-nal control is a viable alternative (i.e., that one or more signalwarrants have been satisfied). Flash mode should be consid-ered during each hour of the average day for which the majorand minor volumes fall in the unshaded region. If flash modeis used, it should be used during extended periods (i.e., severalhours per period) each day to minimize driver confusion.

Application of this guideline requires two types of data:

1. Major-road and minor-road approach volumes for eachhour of the average day and

2. Major-road and minor-road approach through-lanecount.

These traffic demands can be measured, or they can be esti-mated as a fraction of the average daily traffic demand.Kacir et al. (12) suggest the values listed in Table 2-6 canbe used to estimate demands during the late-night hours.Also, the stopped drivers’ unobstructed view of approach-ing unstopped vehicles should be verified before flash modeis implemented.

Convert to Traffic Signal Control

Introduction. An intersection with a properly designedand operated traffic control signal will have one or more ofthe following benefits (relative to an intersection without atraffic signal):

0

50

100

150

200

250

300

0 200 400 600 800 1000

Major R oadw ay (total o f b oth ap proaches), veh /h

Min

or

Ro

ad

wa

y(t

ota

l of

bo

th a

pp

roa

ch

es

), v

eh

/h

Consider yellow/red flash mode.

2/1 & 2/2

2/2

Yellow/red

safe but

may cause

delay.

Do not cons ider

yellow/red flash mode.

1/1, 2/1, 2/2

Legend: x /y - x = major road lanes, y = minor road lanes

Figure 2-2. Guideline for use of signal flash mode.

• More orderly movement of traffic,• Increased intersection capacity,• Reduced frequency of certain types of collisions (e.g.,

right-angle),• Continuous or nearly continuous movement of traffic

along the through route, and• Reduced delay to minor vehicular and pedestrian

movements by interrupting heavy traffic at periodicintervals.

If the traffic signal is not properly designed or operated, oneor more of the following problems may result:

1. Excessive delay to all traffic movements,2. Excessive disobedience of the signal indication,3. Increased frequency of diversion through neighbor-

hoods, and4. Increased frequency of certain types of collisions (e.g.,

rear-end).

These four problems may also be evident at a signalizedintersection whose traffic signal is no longer needed.

Guidance. The MUTCD 2000 describes eight warrantsthat define conditions in which a traffic control signal is likelyto improve intersection safety, operations, or both. Thesewarrants are listed in Table 2-7. If a warrant is met, then sig-nal control may be appropriate. However, the appropriatenessof the signal should be confirmed through the conduct of anengineering study (as described in Chapter 3).

Application. Collectively, the warrants listed in Table 2-7address a wide range of factors that can affect safety and oper-ations at an intersection. These factors include traffic vol-

15

umes, volume variations during the day, approach geometry,major-road speed, crash frequency, motorist delay, and gapfrequency in the major-road traffic stream. Often, only a sub-set of these warrants are evaluated because only one or two warrants are likely to be sensitive to the problem beingexperienced at the subject intersection.

Once the warrants to be evaluated are identified, the nec-essary data are collected using appropriate field study tech-niques or estimation methods. The data needed to evaluateeach warrant are listed in Table 2-8. Procedures for collect-ing these data are described in the Manual of TransportationEngineering Studies (6 ). Data collection procedures are alsodescribed in “Traffic Signal Warrants: Guidelines for Con-ducting a Traffic Signal Warrant Analysis” (7 ). For pro-posed intersections, techniques described in Appendix C canbe used to estimate turn movement volumes from forecasttraffic demands.

Convert to Multi-Way Stop Control

Introduction. Multi-way stop control is most useful whenintersection traffic volume is high enough to create frequentconflicts and the traffic volume is evenly split between theintersecting roads. Multi-way stop control may reduce thenumber of crashes more than will other, less-restrictive formsof control. Multi-way stop control can also result in less delaythan other types of control when approach demands are nearlybalanced and do not satisfy one or more of the MUTCD 2000Warrants 1, 2, or 3.

Guidance. The MUTCD 2000 (1, p. 2B-10) describes fourcriteria that define conditions in which a multi-way stop islikely to improve intersection safety, operations, or both. If one

TABLE 2-6 Hourly volume levels expressed as a percentage of average daily traffic demand

TABLE 2-7 Signal warrants in the MUTCD 2000

criterion is met, multi-way stop control may be appropriate.However, this finding should be confirmed through the con-duct of an engineering study (as described in Chapter 3). Thefour guidance criteria are as follows:

A. Where traffic control signals are justified, the multi-way stop is an interim measure that can be installedquickly to control traffic while arrangements arebeing made for the installation of the traffic controlsignal.

B. A crash problem, as indicated by five or more reportedcrashes in a 12-month period that are susceptible to cor-rection by a multi-way stop installation. Such crashesinclude right- and left-turn collisions as well as right-angle collisions.

C. Minimum volumes:1. The vehicular volume entering the intersection

from the major-street approaches (total of both ap-proaches) averages at least 300 vehicles per hourfor any 8 hours of an average day, and

2. The combined vehicular, pedestrian, and bicyclevolume entering the intersection from the minorstreet approaches (total of both approaches) aver-ages at least 200 units per hour for the same 8 hours,with an average delay to minor-street vehicular traf-fic of at least 30 seconds per vehicle during thehighest hour, but

3. If the 85th percentile approach speed of the major-street traffic exceeds 65 km/h (40 mph), the mini-mum vehicular volume warrants are 70 percent ofthe above values.

D. Where no single criterion is satisfied, but where Crite-ria B, C.1, and C.2 are all satisfied to 80 percent of theminimum values. Criterion C.3 is excluded from thiscondition.

Option:

16

Other criteria that may be considered in an engineeringstudy include:

A. The need to control left-turn conflicts.B. The need to control vehicle/pedestrian conflicts near

locations that generate high pedestrian volumes.C. Locations where a road user, after stopping, cannot

see conflicting traffic and is not able to safely nego-tiate the intersection unless conflicting cross traffic isalso required to stop.

D. An intersection of two residential neighborhood col-lector (through) streets of similar design and oper-ating characteristics where multi-way stop controlwould improve traffic operational characteristics ofthe intersection.

Application. Collectively, the criteria listed in the pre-ceding section address a wide range of factors that can affectsafety and operations at an intersection. These factors in-clude traffic volume, volume variations during the day, ap-proach geometry, major-road speed, crash frequency, andmotorist delay.

Once the criteria to be evaluated are identified, the nec-essary data are collected using appropriate field study tech-niques or estimation methods. The data needed for each cri-terion are listed in Table 2-9. Procedures for collectingthese data are described in the Manual of TransportationEngineering Studies (6 ).

Convert to Two-Way Stop or Yield Control

Introduction. The Stop sign is intended for intersectionapproaches on which drivers need to stop before proceeding

TABLE 2-8 Data needed to evaluate the MUTCD 2000 signal warrants

into the intersection. A benefit of stop control (over no con-trol) is that it can improve overall intersection safety byclearly defining which movements have to yield the right-of-way. On the other hand, stop control has some disadvantagesthat include the following:

• Increased frequency of some collisions (e.g., rear-endcollision),

• Increased road-user costs through increased fuel con-sumption and delay, and

• Increased air and noise pollution.

The Yield sign is used to inform drivers approaching anintersection that they do not have priority at the intersection butthat stopping is not necessary if they can verify that the inter-section will be clear when they reach it. Yield control is lessrestrictive than stop control, but more restrictive than “no con-trol.” Yield control tends to be associated with more collisionsthan stop control, but has a significantly lower road-user cost.

Guidance. The MUTCD 2000 (1, p. 2B-8) presents fourconditions where stop control may be appropriate:

A. Intersection of a less important road with a main roadwhere application of the normal right-of-way rule wouldnot be expected to provide reasonably safe operation.

B. Street entering a through highway or street.C. Unsignalized intersection in a signalized area.D. High speeds, restricted view, or crash records indicates

a need for control by the STOP sign.

The MUTCD 2000 (1, 2B-12) also offers several condi-tions where yield control may be appropriate. The conditionsapplicable to intersections are as follows:

A. When the ability to see all potentially conflicting traf-fic is sufficient to allow a road user traveling at theposted speed, the 85th percentile speed, or the statutoryspeed to pass through the intersection or to stop in asafe manner.

. . . . . .

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C. At the second crossroad of a divided highway, wherethe median width is 9 m (30 ft) or greater. A STOP signmay be installed at the entrance to the first roadway ofa divided highway and a YIELD sign may be installedat the entrance to the second roadway.

D. At an intersection where a special problem exists andwhere engineering judgment indicates the problem tobe susceptible to correction by the use of the Yield sign.

With regard to the second Condition “A” above (i.e.,“When the ability. . . .”), the minor-road driver’s view ofthe major road should not be obstructed by curvature,grade, or objects in one or both of the quadrants adjacent tothe minor-road approach. Guidelines for determining theminor-road driver’s sight distance needs are described inChapter 5 of the Manual of Transportation EngineeringStudies (6 ).

Box (14) developed guidelines for use of traffic controlsigns at low-volume urban intersections. He recommendedconsideration of roadway classification, crash history, andthe safe approach speed in determining the most appropriatecontrol mode. The recommendations made by Box have beenincorporated in Table 2-10.

Box (14) indicates that Table 2-10 should only be used forintersections with a total entering traffic volume of 300 veh/hor less during the peak hour. He also cautions that the no-control or yield-control options may not work well when thetotal entering volume exceeds 100 veh/h.

Application. The guidance stated in the preceding sec-tion indicates the conditions suitable to the use of two-waystop or yield control. Using this guidance requires five typesof data:

1. Major- and minor-road approach volumes for the peakhour of the average day;

2. Major-road 85th percentile speed (posted speed can besubstituted if data are unavailable);

3. Minor-road safe approach speed (based on availableintersection sight distance);

TABLE 2-9 Data needed to evaluate the MUTCD 2000 multi-way stop control criteria

4. Major- and minor-road classification; and5. Crash history for the previous 12-months as a minimum,

but preferably for the previous 3 years.

These data would be used with Table 2-10 to determine themost appropriate minor-road control mode. The conditionsfrom the MUTCD 2000 listed (see the preceding Guidancesection) should be assessed in combination with Table 2-10.If any of these conditions appear satisfied or if the conclusionreached from the use of Table 2-10 indicates that stop or yieldcontrol is appropriate, then stop or yield control may be aviable alternative for controlling the minor-road intersectionapproach.

Prohibit On-Street Parking

Introduction. Parking maneuvers into or out of on-streetparking stalls can affect the operation and safety of thethrough traffic lane adversely. The stalls can be either angleor parallel with the curb. The time required for the parkingmaneuver is shorter for the angled stall than for a parallelparking stall. However, when leaving the stall, it is faster toleave a parallel parking stall than to leave an angled stall.Measurements indicate that the total blockage for both man-euvers is about 36 s. Chapter 16 of the Highway CapacityManual (15) indicates that such maneuvers momentarilyblock the adjacent through lane and can significantly reduce

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intersection capacity if they occur within 76 m (250 ft) of thestop line.

Parking on the intersection approach can also have safetyconsequences. Cleveland et al. (16) report two studies indi-cating that the removal of parking can reduce crash frequencyby 16 to 32 percent. Parked vehicles along one road can alsocreate safety problems for drivers on the intersecting road byblocking the driver’s view of conflicting traffic.

Guidance. Special Report 125: Parking Principles (17)describes guidelines for determining when to prohibit on-street parking. These guidelines indicate maximum flow ratesthat can be associated with on-street parking. Table 2-11summarizes the guidance provided.

Application. The guidance described in the precedingsection describes conditions that may justify the prohibitionof on-street parking. Use of this guidance requires two typesof data:

1. Major- and minor-road approach volumes for 8 or morehours of the average day.

2. Major- and minor-road approach through-lane count.

Table 2-11 should be consulted once for each hour of inter-est on a given intersection approach. Each approach is indi-vidually evaluated. If the combination of volume and lanes

TABLE 2-10 Candidate control for the minor-road approach1

TABLE 2-11 Guidelines for determining when to prohibit on-street parking

exceeds the maximum value listed in the table, then parkingprohibition should be considered a viable alternative for thesubject approach during the specified hour. If it is determinedthat parking should be prohibited during any 1 hr, it wouldbe preferable to prohibit parking during several hours (e.g.,7:00 a.m. to 6:00 p.m.) to minimize enforcement problems.However, parking prohibition during just the peak trafficdemand hours can be considered.

Prohibit Left-Turn Movements

Introduction. Left-turning vehicles at an unsignalizedintersection can create numerous operational and safety prob-lems. The left-turn maneuver tends to require a longer servicetime than the through maneuver. In fact, even modest left-turn volumes can cause safety or operational problems, espe-cially when there is inadequate storage for left-turn vehicles.When the right-of-way needed to provide this storage is notavailable, left-turn restriction (through regulation or chan-nelization) is a means of eliminating these problems. How-ever, the potential benefits of turn restriction should be care-fully weighed against the increased travel time and trip lengththat is likely to be incurred by redirected motorists.

Guidance. Turn restrictions at an intersection should becarefully considered because they can cause traffic to divertto other, local roads. In general, the analyst should ensurethat turn restriction is part of a larger program intended toidentify the ultimate cause of the left-turn problem. Causesof such problems may include inadequate left-turn capacityat adjacent signalized intersections, inadequate left-turn stor-age at the subject intersection, and inadequate arterial travelspeed (such that traffic diverts through neighborhoods via aleft-turn).

The following guidelines can be used to identify the con-ditions suitable for left-turn restriction at existing intersec-tions. These guidelines are based on the criteria offered byKoepke and Levinson (18).

• Left-turn-related delay, conflicts, or crash frequencyshould be at unacceptable levels.

• An alternative route is available for the redirected left-turn vehicles.

• The alternative route is not expected to add more than afew minutes to the redirected motorist’s travel time.

• The intersection is in an urban or suburban area. (Note: insuburban settings, turn restriction is generally not foundexcept where such treatments are part of an areawidecirculation plan.)

If operational problems are only experienced during thepeak hours, then left-turn restriction during these hours maybe a viable alternative. If operational problems exist through-out the day and provision of a left-turn bay (of adequatelength) is not an option, then full-time left-turn restriction

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may be a viable alternative. Regardless of the duration of therestriction, all four of the above criteria should be satisfiedbefore turn restriction is given further consideration.

Application. The guidance described in the precedingsection can be used to determine if left-turn restriction mightimprove operations or safety at an intersection. Evaluationof this guidance requires three types of data (as measuredduring the morning peak hour, afternoon peak hour, and onerepresentative off-peak hour of the average day):

1. Delay resulting from left-turn vehicles queued in athrough lane because of nonexistent or inadequate baystorage;

2. Left-turn-related traffic conflicts (or left-turn-relatedcrash history for the previous year); and

3. Travel time for the likely alternative route(s).

The left-turn-related delay, conflict, and crash data shouldbe used to determine if turn restriction is needed and, ifneeded, for what hours of the day. When turn restrictions onlyneed to be in place during certain times of the day, the turnrestriction should be shown by (1) a variable message sign,(2) an internally illuminated sign whose legend is visibleonly during the hours where the prohibition is applicable, or(3) permanently mounted signs with a supplementary legendstating the hours when the prohibition is in effect. If turnrestriction is needed throughout the day, then raised-curbchannelization should be considered to maximize compliance.

Convert to Roundabout

Introduction. The modern roundabout can offer opera-tional and safety benefits that exceed those offered by otherforms of intersection control for certain conditions. Robin-son et al. (19) indicate that roundabouts have the followingadvantages:

1. They are effective traffic-calming devices because theyreduce speeds.

2. They can reduce the frequency and severity of somecrashes (e.g., right-angle, head-on, and turn-related).

3. They result in less delay than multi-way stop control.4. They result in less delay than two-way stop control when

volumes are sufficient to cause operational problems atthe two-way stop-controlled intersection.

5. They result in less delay than signal control when vol-umes do not exceed the roundabout’s capacity.

The roundabout also has some characteristics that may pre-clude its use at some locations. These characteristics are asfollows:

1. Roundabouts may require more right-of-way than aconventional intersection.

2. They may not be easily traversed by large or over-sizedtrucks.

3. Roundabouts may not yield efficient operation if themajor-road volume greatly exceeds the minor-roadvolume.

4. They may not be conducive to through movement pro-gression in coordinated signal networks.

5. They may not be conducive to serving pedestrian orbicycle traffic.

Guidance. The report prepared by Robinson et al. (19)provides some information on the conditions where modernroundabouts are well suited. These conditions have been re-stated in Table 2-12 in terms of questions to be answered whenconsidering a roundabout. The questions have been worded sothat the roundabout becomes a more viable alternative as thenumber of questions answered “Yes” increases.

The maximum daily service volume needed to answerQuestion 2 in Table 2-12 can be obtained from Figure 2-3.This figure was developed by Robinson et al. (19) and as-sumes that (1) the peak hour has 10 percent of the daily vol-ume, (2) one direction of flow on each road has 58 percent ofthe total two-way flow, (3) 10 percent of each approach vol-ume is turning right, and (4) the maximum service volumeequates to 85 percent of capacity. Figure 2-3 applies to a four-leg roundabout; if applied to a three-leg roundabout, the val-ues obtained from the figure should