Design Guidelines for Horizontal Sightline Offsets (2019) / Chapter Skim
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3 1.1 Introduction SSD is an important geometric design element -- one of the FHWA controlling criteria for geometric design -- and widely accepted design criteria for SSD are presented in the AASHTO's A Policy on Geometric Design for Highways and Streets, commonly known as the Green Book (AASHTO 2011)
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4 Design Guidelines for Horizontal Sightline Offsets Generations of highway engineers have been taught that SSD is needed at all points along the roadway alignment. Yet, despite this perceived importance, there are no definitive crash modification factors (CMFs)
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Introduction 5 where a traffic barrier or a bridge rail on an elevated structure serves as a horizontal sight obstruction, removing the sight obstruction could cost hundreds of thousands, if not millions, of dollars. Such decisions are complex because some horizontal sight obstructions may be critical to crash reduction, and should therefore be removed; at other locations, leaving the sight obstruction in place may be highly unlikely to lead to collisions and thus may not merit additional large investments.
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6 Design Guidelines for Horizontal Sightline Offsets One of the situations with horizontal sight distance restrictions that highway agencies find most difficult and expensive to deal with occurs on a freeway or a ramp curving either to the right or to the left with a barrier on the inside of the curve. In this situation, the barrier itself can become a sight obstruction; the barrier may be very expensive to reposition or remove, and may be critical to roadside safety in its current location.
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Introduction 7 curve to the right serve as horizontal sight obstructions. And, in both cases, sight distance is also limited for drivers in the opposing direction of travel traversing a curve to the left.
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8 Design Guidelines for Horizontal Sightline Offsets (a)
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Introduction 9 1.3 Organization of This Guide This guide presents the results of a literature review and highway agency survey, a set of representative design scenarios faced by highway agencies, the results of data collection and analysis conducted as part of the research, the development of a reliability analysis model for horizontal sight distance, and design guidelines for highway agencies. The remainder of this guideline document is organized as follows: Chapter 2 -- Design Criteria for HSOs Chapter 3 -- Relationship of Sight Distance to Crash Frequency and Severity Chapter 4 -- Benefit-Cost Analysis Chapter 5 -- Reliability Analysis Model for Horizontal Curves with Limited SSD Chapter 6 -- Assessing Removal or Mitigation of Horizontal Sight Obstructions Chapter 7 -- Design Exceptions and Mitigation Strategies References Appendix A -- Computation of HSOs Appendix B -- Users Guide for Reliability Analysis Tool Appendix C -- Case Studies of Existing Roadways with Sight Obstructions
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