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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2019. Design Guidelines for Horizontal Sightline Offsets. Washington, DC: The National Academies Press. doi: 10.17226/25537.
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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2019. Design Guidelines for Horizontal Sightline Offsets. Washington, DC: The National Academies Press. doi: 10.17226/25537.
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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2019. Design Guidelines for Horizontal Sightline Offsets. Washington, DC: The National Academies Press. doi: 10.17226/25537.
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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2019. Design Guidelines for Horizontal Sightline Offsets. Washington, DC: The National Academies Press. doi: 10.17226/25537.
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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2019. Design Guidelines for Horizontal Sightline Offsets. Washington, DC: The National Academies Press. doi: 10.17226/25537.
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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2019. Design Guidelines for Horizontal Sightline Offsets. Washington, DC: The National Academies Press. doi: 10.17226/25537.
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2019 N A T I O N A L C O O P E R A T I V E H I G H W A Y R E S E A R C H P R O G R A M NCHRP RESEARCH REPORT 910 Design Guidelines for Horizontal Sightline Offsets Ingrid B. Potts Douglas W. Harwood Daniel J. Cook MRIGlobal Kansas City, MO Eric T. Donnell Bachir Hamadeh laRson PennsylvanIa TRansPoRTaTIon InsTITuTe University Park, PA Subscriber Categories Highways • Design • Safety and Human Factors Research sponsored by the American Association of State Highway and Transportation Officials in cooperation with the Federal Highway Administration

NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM Systematic, well-designed, and implementable research is the most effective way to solve many problems facing state departments of transportation (DOTs) 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 transporta- tion results in increasingly complex problems of wide interest to high- way 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 ini- tiated 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, 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 TRB’s recognized objectivity and understanding of modern research practices. TRB is uniquely suited for this purpose for many reasons: TRB maintains an extensive com- mittee 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, univer- sities, and industry; TRB’s relationship to the National Academies is an insurance of objectivity; and TRB maintains a full-time staff of special- ists 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 Federal Highway Administration. Topics of the highest merit are selected by the AASHTO Special Committee on Research and Innovation (R&I), and each year R&I’s recommendations are proposed to the AASHTO Board of Direc- tors 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 surveillance of research contracts are the responsibilities of the National Academies and TRB. 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. Published research reports of the NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM are available from Transportation Research Board Business Office 500 Fifth Street, NW Washington, DC 20001 and can be ordered through the Internet by going to http://www.national-academies.org and then searching for TRB Printed in the United States of America NCHRP RESEARCH REPORT 910 Project 15-59 ISSN 2572-3766 (Print) ISSN 2572-3774 (Online) ISBN 978-0-309-48054-3 Library of Congress Control Number 2019943868 © 2019 National Academy of Sciences. All rights reserved. COPYRIGHT INFORMATION Authors herein are responsible for the authenticity 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. Cooperative Research Programs (CRP) grants permission to reproduce material in this publication for classroom and not-for-profit purposes. Permission is given with the understanding that none of the material will be used to imply TRB, AASHTO, FAA, FHWA, FMCSA, FRA, FTA, Office of the Assistant Secretary for Research and Technology, PHMSA, or TDC endorsement of a particular product, method, or practice. It is expected that those reproducing the material in this document for educational and not-for-profit uses will give appropriate acknowledgment of the source of any reprinted or reproduced material. For other uses of the material, request permission from CRP. 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. The opinions and conclusions expressed or implied in this report 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; or the program sponsors. 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 manufacturers’ names appear herein solely because they are considered essential to the object of the report.

The National Academy of Sciences was established in 1863 by an Act of Congress, signed by President Lincoln, as a private, non- governmental institution to advise the nation on issues related to science and technology. Members are elected by their peers for outstanding contributions to research. Dr. Marcia McNutt is president. 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. C. D. Mote, Jr., is president. The National Academy of Medicine (formerly the Institute of Medicine) was established in 1970 under the charter of the National Academy of Sciences to advise the nation on medical and health issues. Members are elected by their peers for distinguished contributions to medicine and health. Dr. Victor J. Dzau is president. The three Academies work together as the National Academies of Sciences, Engineering, and Medicine to provide independent, objective analysis and advice to the nation and conduct other activities to solve complex problems and inform public policy decisions. The National Academies also encourage education and research, recognize outstanding contributions to knowledge, and increase public understanding in matters of science, engineering, and medicine. Learn more about the National Academies of Sciences, Engineering, and Medicine at www.national-academies.org. The Transportation Research Board is one of seven major programs of the National Academies of Sciences, Engineering, and Medicine. The mission of the Transportation Research Board is to increase the benefits that transportation contributes to society by providing leadership in transportation innovation and progress through research and information exchange, conducted within a setting that is objective, interdisciplinary, and multimodal. The Board’s varied committees, task forces, and panels annually engage about 7,000 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. Learn more about the Transportation Research Board at www.TRB.org.

C O O P E R A T I V E R E S E A R C H P R O G R A M S CRP STAFF FOR NCHRP RESEARCH REPORT 910 Christopher J. Hedges, Director, Cooperative Research Programs Lori L. Sundstrom, Deputy Director, Cooperative Research Programs William C. Rogers, Senior Program Officer Jarrel McAfee, Senior Program Assistant Eileen P. Delaney, Director of Publications Natalie Barnes, Associate Director of Publications Margaret B. Hagood, Editor NCHRP PROJECT 15-59 PANEL Field of Design—Area of General Design Trinity D. Smith, Arkansas DOT, Little Rock, AR (Chair) Zhongren Wang, Division of Maintenance, Caltrans, Sacramento, CA Andrew Jay Beagle, City of Olympia, Public Works, Olympia, WA Keith R. Damron, American Engineers, Inc., Louisville, KY Sonia Alicia Mercado, Texas DOT, Austin, TX Jason R. Salley, Illinois DOT, Schaumburg, IL Michael Matzke, FHWA Liaison Stephen F. Maher, TRB Liaison

NCHRP Research Report 910: Design Guidelines for Horizontal Sightline Offsets provides guidance for highway agencies to address the types of sight distance restrictions that are most likely to be encountered on specific roadway types. They identify design situations that address the relative cost of removing the sight restriction and the likely implications for safety of allowing the sight distance restriction to remain. When a decision is reached not to remove a sight restriction, the guidelines include a catalog of mitigation strategies for consideration in specific design situations on specific roadway types. When the sightlines along horizontal roadway curves are restricted, there are challenges to find acceptable solutions. Designers compensate for the limitations on driver sight distance in various ways, including: accepting shorter sightlines, lowering design speed, increasing shoulder width, or providing additional signage. There are advantages and disadvantages to the trade-offs; as a result, many highway agencies have used the design exception process to address the trade-offs for sight distance in such situations. This project conducted research to evaluate these situations and determine what criteria or mitigation will provide acceptable solutions when impaired horizontal sightline offsets are encountered. In NCHRP Project 15-59, MRIGlobal, assisted by Larson Pennsylvania Transportation Institute, was asked to: (1) evaluate the safety and operational performance as well as the trade-offs and risks associated with state-of-the-practice mitigation treatments when horizontal sightline offset criteria are not met; and (2) recommend updates to AASHTO’s A Policy on Geometric Design of Highways and Streets (Green Book) for horizontal sightline offset design criteria, and guidance on curved roadway alignment adjacent to barriers or other types of impediments that may impact the driver’s line of sight. F O R E W O R D By William C. Rogers Staff Officer Transportation Research Board

1 Summary 3 Chapter 1 Introduction 3 1.1 Introduction 6 1.2 Typical Locations with Horizontal Sight Obstructions 9 1.3 Organization of This Guide 10 Chapter 2 Design Criteria for HSOs 10 2.1 Current SSD Design Criteria 14 2.2 Components of AASHTO SSD Criteria 15 2.3 HSO Design Criteria 20 2.4 Alternative Assumptions for Measuring Horizontal Sight Distance 21 Chapter 3 Relationship of Sight Distance to Crash Frequency and Severity 23 Chapter 4 Benefit-Cost Analysis 24 4.1 Crash Reduction for Sight Distance Improvements 24 4.2 Expected Annual Crash Frequency 24 4.3 Benefit per Crash Reduced 24 4.4 Improvement Service Life 24 4.5 Discount Rate or Minimum Attractive Rate of Return 25 4.6 Uniform Series Present Worth Factor 25 4.7 Project Implementation Cost 25 4.8 Cost-Effectiveness Analysis 26 4.9 Computational Example 28 Chapter 5 Reliability Analysis Model for Horizontal Curves with Limited SSD 29 5.1 Input Data for the Reliability Analysis Model 30 5.2 Output Data from the Reliability Analysis Model 32 5.3 Structure of Reliability Analysis Model 33 5.4 Available SSD Module 35 5.5 Reliability Module 39 5.6 Application of Reliability Analysis to Curves with Horizontal Sight Obstructions 44 Chapter 6 Assessing Removal or Mitigation of Horizontal Sight Obstructions 44 6.1 Step-by-Step Procedure for Assessing Whether to Remove or Mitigate Existing Horizontal Sight Obstructions 47 6.2 Assessment of Horizontal Sight Obstructions Under Design for New Construction 47 6.3 Case Studies of Existing Roadways with Horizontal Sight Obstructions C O N T E N T S

49 Chapter 7 Design Exceptions and Mitigation Strategies 49 7.1 Design Exceptions 49 7.2 Potential Mitigation Strategies for Limited Horizontal Sight Distance 57 References 59 Appendix A Computation of HSOs 71 Appendix B Users Guide for Reliability Analysis Tool 82 Appendix C Case Studies of Existing Roadways with Sight Obstructions Note: Photographs, figures, and tables in this report may have been converted from color to grayscale for printing. The electronic version of the report (posted on the web at www.trb.org) retains the color versions.

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The distance between the driver’s line of sight along the roadway ahead on a horizontal curve and a sight obstruction on the inside of the curve is known as the horizontal sightline offset (HSO). Highway agencies can use NCHRP Research Report 910: Design Guidelines for Horizontal Sightline Offsets as guidance to address the types of sight distance restrictions that are most likely to be encountered on specific roadway types.

The relationship between stopping sight distance (SSD) and the frequency and severity of crashes has been difficult to quantify because the role of SSD in reducing crashes is highly situational. The design criteria for the horizontal component of SSD in what is known as AASHTO's Green Book are based on the maximum sightline offset that may be needed at any point along a curve with a given radius, which doesn't cover all possible situations.

Designers compensate for the limitations on driver sight distance in various ways, including: accepting shorter sightlines, lowering design speed, increasing shoulder width, or providing additional signage. There are advantages and disadvantages to the trade-offs; as a result, many highway agencies have used the design exception process to address the trade-offs for sight distance in such situations.

This project conducted research to evaluate these situations and determine what criteria or mitigation will provide acceptable solutions when impaired horizontal sightline offsets are encountered. The project includes a tool (an Excel spreadsheet) that may be used to calculate sight distance.

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