Human Factors Guidelines for Road Systems: Third Edition (2025)
Chapter: Front Matter
TRANSPORTATION RESEARCH BOARD 2025 EXECUTIVE COMMITTEE*
OFFICERS
Chair: Leslie S. Richards, Professor of Practice, University of Pennsylvania, Philadelphia
Vice Chair: Joel M. Jundt, Secretary of Transportation, South Dakota Department of Transportation, Pierre
Executive Director: Victoria Sheehan, Transportation Research Board, Washington, DC
MEMBERS
James F. Albaugh, President and CEO, The Boeing Company (retired), Scottsdale, AZ
Carlos M. Braceras, Executive Director, Utah Department of Transportation, Salt Lake City
Douglas C. Ceva, Vice President, Customer Lead Solutions, Prologis, Inc., Jupiter, FL
Nancy Daubenberger, Commissioner of Transportation, Minnesota Department of Transportation, St. Paul
Marie Therese Dominguez, Commissioner, New York State Department of Transportation, Albany
Garrett Eucalitto, Commissioner, Connecticut Department of Transportation, Newington
Andrew Fremier, Executive Director, Metropolitan Transportation Commission, San Francisco, CA
Martha Grabowski, Professor Emerita, Information Systems, Le Moyne College, Madden College of Business & Economics, Cazenovia, NY
Randell Iwasaki, President and CEO, Iwasaki Consulting Services, Walnut Creek, CA
Carol A. Lewis, Professor, Transportation Studies, Texas Southern University, Houston
Scott C. Marler, Director, Iowa Department of Transportation, Ames
Ricardo Martinez, Adjunct Professor of Emergency Medicine, Emory University School of Medicine, Decatur, GA
Russell McMurry, Commissioner, Georgia Department of Transportation, Atlanta
Craig E. Philip, Research Professor and Director, VECTOR, Department of Civil and Environmental Engineering, Vanderbilt University, Nashville, TN
Steward T.A. Pickett, Distinguished Senior Scientist, Cary Institute of Ecosystem Studies, Millbrook, NY
Susan A. Shaheen, Professor and Co-Director, Transportation Sustainability Research Center, University of California, Berkeley
Marc Williams, Executive Director, Texas Department of Transportation, Austin
EX OFFICIO MEMBERS
Bryan Bedford, Administrator, Federal Aviation Administration, Washington, DC
Michael R. Berube, Deputy Assistant Secretary for Sustainable Transportation, U.S. Department of Energy, Washington, DC
Tariq Bokhari, Acting Administrator, Federal Transit Administration, Washington, DC
Steven G. Bradbury, Deputy Secretary, U.S. Department of Transportation, Washington, DC
Steven Cliff, Executive Officer, California Air Resources Board, Sacramento
Drew Feeley, Acting Administrator, Federal Railroad Administration, Washington, DC
LeRoy Gishi, Chief, Division of Transportation, Bureau of Indian Affairs, U.S. Department of the Interior, Germantown, MD
Firas Ibrahim, Director, Office of Research, Development, and Technology, Office of the Assistant Secretary for Research and Technology (OST-R), Washington, DC
Jason Kelly, Deputy Commanding General for Civil Works and Emergency Operations, U.S. Army Corps of Engineers, Washington, DC
Sandra Knight, President, WaterWonks, LLC, Washington, DC
Ben Kochman, Acting Administrator, Pipeline and Hazardous Materials Safety Administration, Washington, DC
Niloo Parvinashtiani, Engineer, Mobility Consultant Solutions, Iteris Inc., Fairfax, VA, and Chair, TRB Young Members Coordinating Council
Gloria Shepherd, Acting Deputy Administrator, Federal Highway Administration, Washington, DC
Karl Simon, Director, Transportation and Climate Division, U.S. Environmental Protection Agency, Washington, DC
Paul P. Skoutelas, President and CEO, American Public Transportation Association, Washington, DC
Jim Tymon, Executive Director, American Association of State Highway and Transportation Officials, Washington, DC
Sang Yi, Acting Administrator, U.S. Maritime Administration, Washington, DC
________________________
* Membership as of August 2025.
NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM
NCHRP RESEARCH REPORT 1148
Human Factors Guidelines for Road Systems
THIRD EDITION
John L. Campbell
Liberty Hoekstra-Atwood
Exponent, Inc.
Seattle, WA
Audra Fraser
Exponent, Inc.
Irvine, CA
Chris Monk
Exponent, Inc.
Washington, DC
James L. Brown
Joonbum Lee
Monica G. Lichty
David M. Prendez
Christian M. Richard
Battelle Memorial Institute
Seattle, WA
Alicia Romo
Battelle Memorial Institute
Washington, DC
Ingrid Potts
Texas A&M Transportation Institute
Leeʼs Summit, MO
Darren Torbic
Texas A&M Transportation Institute
State College, PA
Jerry Graham
Douglas Harwood
Jessica Hutton
Mitchell OʼLaughlin
Midwest Research Institute
Kansas City, MO
Subscriber Categories
Design • Operations and Traffic Management • Safety and Human Factors
Research sponsored by the American Association of State Highway and Transportation Officials in cooperation with the Federal Highway Administration
2025
NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM
Systematic, well-designed, and implementable research is the most effective way to solve many problems facing state department of transportation (DOT) administrators and engineers. Often, highway problems are of local or regional interest and can best be studied by state DOTs individually or in cooperation with their state universities and others. However, the accelerating growth of highway transportation results in increasingly complex problems of wide interest to highway authorities. These problems are best studied through a coordinated program of cooperative research.
Recognizing this need, the leadership of the American Association of State Highway and Transportation Officials (AASHTO) in 1962 initiated an objective national highway research program using modern scientific techniques—the National Cooperative Highway Research Program (NCHRP). NCHRP is supported on a continuing basis by funds from participating member states of AASHTO and receives the full cooperation and support of the Federal Highway Administration (FHWA), United States Department of Transportation.
The Transportation Research Board (TRB) of the National Academies of Sciences, Engineering, and Medicine was requested by AASHTO to administer the research program because of TRBʼs recognized objectivity and understanding of modern research practices. TRB is uniquely suited for this purpose for many reasons: TRB maintains an extensive committee structure from which authorities on any highway transportation subject may be drawn; TRB possesses avenues of communications and cooperation with federal, state, and local governmental agencies, universities, and industry; TRBʼs relationship to the National Academies is an insurance of objectivity; and TRB maintains a full-time staff of specialists in highway transportation matters to bring the findings of research directly to those in a position to use them.
The program is developed on the basis of research needs identified by chief administrators and other staff of the highway and transportation departments, by committees of AASHTO, and by the FHWA. Topics of the highest merit are selected by the AASHTO Special Committee on Research and Innovation (R&I), and each year R&Iʼs recommendations are proposed to the AASHTO Board of Directors, the FHWA, and the National Academies. Research projects to address these topics are defined by NCHRP, and qualified research agencies are selected from submitted proposals. Administration and oversight of research contracts are the responsibilities of NCHRP.
The needs for highway research are many, and NCHRP can make significant contributions to solving highway transportation problems of mutual concern to many responsible groups. The program, however, is intended to complement, rather than to substitute for or duplicate, other highway research programs.
NCHRP RESEARCH REPORT 1148
Project 22-46
ISSN 2572-3766 (Print)
ISSN 2572-3774 (Online)
ISBN 978-0-309-99386-9
Library of Congress Control Number 2025943236
© 2025 by the National Academy of Sciences. National Academies of Sciences, Engineering, and Medicine and the graphical logo are trademarks of the National Academy of Sciences. All rights reserved.
COPYRIGHT INFORMATION
Authors herein are responsible for the originality and accuracy of their materials and for obtaining written permissions from publishers or persons who own the copyright to any previously published or copyrighted material used herein.
The National Academy of Sciences (NAS) grants permission to reproduce written material in this publication for classroom and non-commercial purposes subject to the rights of any third parties and appropriate attribution. Permission is given with the understanding that none of the material will be used to imply NAS, TRB, AASHTO, APTA, FAA, FHWA, FTA, GHSA, or NHTSA endorsement of a particular product, method, or practice. For other uses of the written material, users must request permission from the National Academies Press.
NOTICE
The research report was reviewed by the technical panel and accepted for publication according to procedures established and overseen by the Transportation Research Board and approved by the National Academies of Sciences, Engineering, and Medicine.
This material is based upon work supported by the FHWA under Agreement No. 693JJ32350025. Any opinions, findings, and conclusions or recommendations expressed or implied in this document are those of the researchers who performed the research and are not necessarily those of the Transportation Research Board; the National Academies of Sciences, Engineering, and Medicine; the FHWA; or the program sponsors.
The Transportation Research Board does not develop, issue, or publish standards or specifications. The Transportation Research Board manages applied research projects which provide the scientific foundation that may be used by Transportation Research Board sponsors, industry associations, or other organizations as the basis for revised practices, procedures, or specifications.
The Transportation Research Board; the National Academies of Sciences, Engineering, and Medicine; and the sponsors of the National Cooperative Highway Research Program do not endorse products or manufacturers. Trade or manufacturersʼ names or logos appear herein solely because they are considered essential to the object of the report.
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NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM
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The National Academy of Engineering was established in 1964 under the charter of the National Academy of Sciences to bring the practices of engineering to advising the nation. Members are elected by their peers for extraordinary contributions to engineering. Dr. Tsu-Jae Liu is president.
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Learn more about the National Academies of Sciences, Engineering, and Medicine at www.nationalacademies.org.
The Transportation Research Board is one of seven major program divisions of the National Academies of Sciences, Engineering, and Medicine. The mission of the Transportation Research Board is to mobilize expertise, experience, and knowledge to anticipate and solve complex transportation-related challenges. The Boardʼs varied activities annually engage about 8,500 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 departments of transportation, federal agencies including the component administrations of the U.S. Department of Transportation, and other organizations and individuals interested in the development of transportation.
Learn more about the Transportation Research Board at www.TRB.org.
COOPERATIVE RESEARCH PROGRAMS
CRP STAFF FOR NCHRP RESEARCH REPORT 1148
Monique R. Evans, Director, Cooperative Research Programs
Waseem Dekelbab, Deputy Director, Cooperative Research Programs, and Manager, National Cooperative Highway Research Program
David M. Jared, Senior Program Officer
Dajaih Bias-Johnson, Senior Program Assistant
Natalie Barnes, Director of Publications
Heather DiAngelis, Associate Director of Publications
Brendan Foht, Senior Editor
NCHRP PROJECT 22-46 PANEL
Field of Design—Area of Vehicle Barrier Systems
Martha Alicia Brown, Illinois Department of Transportation, Springfield, IL (Chair)
Paul J. Carlson, Automated Roads, Greensboro, NC
John C. Milton, Washington State Department of Transportation, Olympia, WA
Deirdre T. Nash, New Hampshire Department of Transportation, Concord, NH
Bernadette Estioko Phelan, Phelan International LLC, Scottsdale, AZ
T. Scott Smith, Louisiana State University, Lafayette, LA
Samuel C. Tignor, Virginia Polytechnic Institute and State University, McLean, VA
Rosemarie Anderson, FHWA Liaison
Kelly K. Hardy, AASHTO Liaison
FOREWORD
By David M. Jared
Staff Officer
Transportation Research Board
NCHRP Research Report 1148 presents updated guidelines for integrating human factors (HF) principles into the planning, design, construction, and operation of roadway systems. The guidelines build on previous editions of Human Factors Guidelines for Road Systems by updating chapters as needed and according to priority, adding chapters on new HF topics, and including tutorial materials to promote understanding and use of the guidelines. The guidelines should be of value to state departments of transportation (DOTs) and other transportation agencies seeking to provide safer roadway systems for all users by effectively integrating HF principles into roadway systems decisions.
The design and operation of road facilities to date have not, as a rule, adequately accounted for the interaction between road users and the facilities, particularly users who are most vulnerable to crashes. Behavioral factors should be considered by all involved in the planning, design, construction, and operation of multimodal transportation systems. NCHRP Report 600: Human Factors Guidelines for Road Systems, Second Edition (HFG2), is a tool for integrating road user capabilities and limitations into roadway systems decisions. The HFG2 provides HF principles and findings for consideration by highway designers, traffic engineers, and other safety practitioners. Considerable research has been completed since the publication of the HFG2 in 2012 that adds tools and design guidelines for enhancing road user safety. NCHRP Web-Only Document 316: Human Factors Guidelines for Road Systems: 2021 Update was published as an update to the HFG2 that addressed HF considerations for bicycle facilities, pedestrian facilities, and roundabouts.
Under NCHRP Project 22-46, “Human Factors Guidelines for Road Systems, Third Edition (HFG3),” Exponent Inc. was asked to prepare HF guidelines that can be used by transportation agencies to integrate HF principles into roadway systems decisions that benefit all road users, including but not limited to drivers, pedestrians, bicyclists, motorcyclists, and transit users. The HFG3 documents the best available HF and road/user interaction research and practices in road safety analyses and design to optimize data-driven safety analysis and decision-making. These practices include the Safe System Approach, wherein roads are designed and operated to reduce the risk of crashes occurring and, if crashes do occur, reduce the severity of injuries. Lastly, the HFG3 provides safety tools and processes that can supplement the AASHTO Highway Safety Manual (HSM), such as road safety audits, operational reviews, performance planning, and crash data analysis for road systems decisions.
In addition to NCHRP Research Report 1148, the following deliverables can be found on the National Academies Press website (nap.nationalacademies.org) by searching for NCHRP Research Report 1148: Human Factors Guidelines for Road Systems, Third Edition.
- A conduct of research report summarizing the entire effort, available as NCHRP Web-Only Document 425: Development of Human Factors Guidelines for Roadway Systems, Third Edition.
- An implementation plan that identifies mechanisms and channels for communicating and implementing this research.
- PowerPoint training modules, supplementary to the tutorials within the HFG3.
CONTENTS
Chapter 1 Why Have Human Factors Guidelines for Road Systems?
The Role of the Human Driver in Roadway Safety
The Purpose of the Human Factors Guidelines for Road Systems
Applying Human Factors to Road System Design and Operations
Scope and Limitations of the HFG
Chapter 2 How to Use This Document
Additional Ways to Use the HFG
P A R T II Bringing Road User Capabilities into Highway Design and Traffic Engineering Practice
Chapter 3 Understanding the Road Userʼs Information-Seeking Behavior
The Road User as a Component of the Highway System
Example Problems of Highway Designers and Traffic Engineers
How Road Users Seek Information
Examples of User-Scanned Road Environments
How Highway Designers and Traffic Engineers Work Together for Road Users
Chapter 4 Integrating Road User, Highway Design, and Traffic Engineering Needs
Iterative Review Steps to Achieve Good Human Factor Applications
Use of Parts III, IV, V, VI, and VII for Aiding Design
P A R T III Human Factors Guidance for Roadway Geometric Elements
Key Components of Sight Distance
Determining When to Use Decision Sight Distance
Determining Passing Sight Distance
Influence of Speed On Sight Distance
Key References for Sight Distance Information
Where to Find Sight Distance Information for Specific Roadway Features
Where to Find Sight Distance Information for Intersections
Chapter 6 Curves (Horizontal Alignment)
Task Analysis of Curve Driving
The Influence of Perceptual Factors on Curve Driving
Countermeasures for Reducing Vehicle Speeds Entering Horizontal Curves
Countermeasures for Improving Steering and Vehicle Control Through Curves
Pavement Surface Countermeasures to Improve Curve Delineation
Warning Signs on Horizontal Curves
Chapter 7 Grades (Vertical Alignment)
Design Considerations for Turnouts on Grades
Geometric and Signing Considerations to Support Effective Use of Truck Escape Ramps
Preview Sight Distance and Grade Perception at Vertical Curves
Chapter 8 Tangent Sections and Roadside (Cross Section)
Task Analysis of Lane Changes on Tangent Sections
Overview of Driver Alertness on Long Tangent Sections
Chapter 9 Transition Zones Between Varying Road Designs
Perceptual and Physical Elements to Support Rural-Urban Transitions
Chapter 10 Non-Signalized Intersections
Factors Affecting Acceptable Gap
Left-Turn Lanes at Non-Signalized Intersections
Sight Distance at Left-Skewed Intersections
Sight Distance at Right-Skewed Intersections
Chapter 11 Signalized Intersections
Engineering Countermeasures to Reduce Red Light Running
Restricting Right Turns on Red to Address Pedestrian Safety
Heuristics for Selecting the Yellow Timing Interval
Countermeasures to Reduce Left-Turn Crashes During Permissive Phases
Reducing Vehicle Speeds Approaching Roundabouts
Increasing Driver Yielding Rates for Pedestrians at Roundabouts
Countermeasures for Improving Accessibility for Visually Impaired Pedestrians at Roundabouts
Task Analysis of Driver Merging Behavior at Freeway Entrance Ramps
Reducing Wrong-Way Entries onto Freeway Exit Ramps
Driver Expectations at Freeway Lane Drops and Lane Reductions
Driver Information Needs at Complex Interchanges
Arrow-per-Lane Sign Design to Support Driver Navigation
Driver Behavioral Trends Based on Exit Ramp Geometry
P A R T IV Special Design Considerations
Chapter 14 Speed Perception, Speed Choice, and Speed Control
Behavioral Framework For Speeding
Speed Perception and Driving Speed
Effects Of Roadway Factors On Speed
Effects of Posted Speed Limits on Speed Decisions
Speeding Countermeasures: Setting Appropriate Speed Limits
Speeding Countermeasures: Communicating Appropriate Speed Limits
Chapter 15 Special Considerations for Urban Environments
Methods to Increase Driver Yielding at Uncontrolled Crosswalks
Methods to Increase Pedestrian Compliance at Uncontrolled Crosswalks
Methods to Reduce Driver Speeds in School Zones
Signage and Markings for High Occupancy Vehicle Lanes
Sight Distance Considerations for Urban Bus Stop Locations
General Design Considerations for Urban Bus Stops
Complete Streets Overview and Heuristics
Complete Streets and Similar Initiatives
A Framework for Selecting Complete Streets Implementations
A Framework for Complete Streets Evaluations
Key References for Complete Streets Design Information
Chapter 17 Special Considerations for Rural Environments
Countermeasures for Pavement/Shoulder Drop-offs
Design Consistency in Rural Driving
Chapter 18 Construction and Work Zones
Chapter 19 Rail-Highway Grade Crossings
Task Analysis of Rail-Highway Grade Crossings
Driver Information Needs at Passive Rail-Highway Grade Crossings
Timing of Active Traffic Control Devices at Rail-Highway Grade Crossings
Four-Quadrant Gate Timing at Rail-Highway Grade Crossings
Countermeasures to Reduce Gate-Rushing at Crossings with Two-Quadrant Gates
Human Factors Considerations in Traffic Control Device Selection at Rail-Highway Grade Crossings
Countermeasures for Mitigating Headlamp Glare
Daytime Lighting Requirements for Tunnel Entrance Lighting
Countermeasures for Improving Pedestrian Conspicuity at Crosswalks
Characteristics of Lighting that Enhance Pedestrian Visibility
Characteristics of Effective Lighting at Intersections
P A R T V Human Factors Guidance for Traffic Engineering Elements
General Principles for Sign Legends
Sign Design to Improve Legibility
Conspicuity of Diamond Warning Signs under Nighttime Conditions
Complexity of Sign Information
Chapter 22 Changeable Message Signs
When to Use Changeable Message Signs
Presentation to Maximize Visibility and Legibility
Determining Appropriate Message Length
Composing a Message to Maximize Comprehension
Displaying Messages with Dynamic Characteristics
Changeable Message Signs for Speed Reduction
Presentation of Bilingual Information
Effectiveness of Symbolic Markings
Markings for Pedestrian and Bicyclist Safety
P A R T VI Human Factors Guidance for Vulnerable and Older Road Users
Task Analysis of Pedestrian Crossing in a Multiple-Threat Scenario
Countermeasures to Reduce Pedestrian Exposure to Vehicles at Crossings
Speed-Calming Countermeasures at Crosswalks
Increasing Pedestrian Visibility and Conspicuity at Crosswalks
Selecting Beacons to Increase Pedestrian Conspicuity at Crosswalks
Influence of the Built Environment on Pedestrian Crossing Safety
Design Challenges for Older Pedestrians
Pedestrian Rail Crossing Safety
Key References for Pedestrian Crossing Safety Countermeasures
Signals and Signal Timing for Bicycles at Intersections
Markings for Bicycles at Intersections
Mitigating Heavy-Vehicle Conflicts with Bicycles
Older Driver Crash Risk, Crash Causation Factors, and Fitness to Drive
Older Driver Considerations for Markings, Signs, and Lighting
Older Driver Considerations for Curves and Merging Behavior
Older Driver Considerations for Work Zones—Signs
Older Driver Considerations for Work Zones—Markings and Channelization
Older Driver Considerations for Intersections—Left Turns
Older Driver Considerations for Intersections—Roundabouts
Older Driver Considerations for Intersections—Signs
Older Driver Considerations for Intersections—Pavement Markings and Signals
P A R T VII Additional Information
Tutorial 1: Real-World Driver Behavior Versus Design Models
Tutorial 2: Diagnosing Sight Distance Problems and Other Design Deficiencies
Tutorial 3: Detailed Task Analysis of Curve Driving
Tutorial 4: Determining Appropriate Clearance Intervals
Tutorial 5: Determining Appropriate Sign Placement and Letter Height Requirements
Tutorial 6: Calculating Appropriate CMS Message Length under Varying Conditions
Tutorial 7: Joint Use of the Highway Safety Manual and the Human Factors Guidelines for Road Systems
