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3 1.1 Introduction SSD is an important geometric design elementâone of the FHWA controlling criteria for geometric designâand widely accepted design criteria for SSD are presented in the AASHTOâs A Policy on Geometric Design for Highways and Streets, commonly known as the Green Book (AASHTO 2011). The vertical component of SSD is fully defined by the vertical profile of the roadway (except on sag vertical curves where an overhead structure may be present). However, the horizontal component of SSD depends on both the vertical and horizontal geometrics of the roadway as well as the varied nature of roadside sight obstructions on the inside of horizontal curves. Sight obstructions can occur on either the right or left side of the roadway, or in a divided highway median. A variety of roadside objects can constitute a horizontal sight obstruction: trees, bushes, utility poles, structures and walls, guardrail, median barrier, rock cuts, or roadside embankments. Figure 1 illustrates a simplified view of the design situation for HSO to roadside sight obstructions, as presented in the Green Book (AASHTO 2011). The figure illustrates a driverâs line of sight to the roadway ahead, which, on a horizontal curve, is a chord of that curve. The dimension labeled HSO in Figure 1, which in terms of analytic geometry is known as the middle ordinate of the curve, is now called the horizontal sightline offset in the Green Book. Any roadside sight obstruction on the inside of the curve whose distance from the centerline of the inside travel lane is less than or equal to HSO may interrupt the driverâs view of the road ahead for at least some portion of the curve. As explained in Section 2.3 of this guide, the dimension labeled HSO in Figure 1 actually represents the maximum HSO that can occur at any point on a horizontal curve; the actual HSO needed at some locations on or near the curve will be less than HSO. The equation for determining HSO for the situation in Figure 1 is: 1 cos 28.65 (1)HSO R S R = â ï£« ï£ï£¬ ï£¶ ï£¸ï£· ï£® ï£°ï£¯ ï£¹ ï£»ï£º where HSO = maximum horizontal sightline offset (ft); R = radius of curve (ft); and S = design value of SSD (ft). The design value of SSD, referred to in the Green Book as S, is known as DSSD when rounded off to a design value. C H A P T E R 1 Introduction
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) that quantify the safety effect of SSD either in the Highway Safety Manual (AASHTO 2010; AASHTO 2014) or on the FHWA CMF Clearinghouse website (www.cmfclearinghouse.org). The likely reason that the safety effects of SSD have not been successfully quantified is that these effects are highly situational, meaning that limited SSD is far more likely to result in a collision at some locations than at others. Recent research in NCHRP Report 783: Evaluation of the 13 Controlling Criteria for Geometric Design (Harwood et al. 2014), found that, at crest vertical curves with limited SSD on rural two-lane highways, crash frequencies were high at locations where intersections, driveways, or horizontal curves were hidden from the approaching driverâs view by the sight restriction. However, where no hidden features were present, crash frequency was not elevated, even though the SSD was limited. On other highway types, such as divided highways and freeways, additional types of hidden features, such as ramp terminals or pedestrian crossings, may also be critical if located in a sight-restricted area. On congested highways, there is a possibility of a standing queue being present in a sight-restricted area during specific time periods during a typical day. The research in NCHRP Report 783 demonstrates for crest vertical curves that correcting or mitigating limited SSD may be much more critical in some highway situations than in others; the same principle is likely to apply to horizontal sight restrictions, as well. Highway agencies face challenges in assessing horizontal sight restrictions in deciding whether to remove the sight restriction, leave it in place, or incorporate mitigation measures, such as signing or a widened shoulder. The difficulty of such decisions increases with the cost of removing the sight obstructions. For example, for some ramps in complex interchanges, Figure 1. Diagram illustrating components for determining horizontal sight distance (AASHTO 2011).
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. Highway agencies currently lack guidelines and/or analysis tools to distinguish between such situations and, therefore, are limited in their ability to make rational design/investment decisions. To address horizontal sight restrictions more effectively in the design process, highway agencies need guidance on the types of sight distance restrictions that are most likely to be encountered on specific roadway types. For specific design situations, guidelines have been developed that address the relative cost of removing the sight restriction and the likely implications for safety of allowing the sight distance restriction to remain. Costs of removing sight restrictions should be addressed in general terms because site-specific features naturally influence costs. The decision of whether to remove a specific sight distance restriction should be addressed, where practical, through economic analysis (i.e., comparison of benefits and costs), which considers the tradeoffs between the crash reduction benefits of removing the sight restriction and the costs of doing so. However, in many cases, it may not be possible to quantify crash reduction benefits of removing the sight obstruction, so alternative estimation methods may need to be used. If a decision is reached not to remove a sight restriction, mitigation measures should be considered; the design guidelines include a catalog of mitigation strategies that can be considered in specific design situations on specific roadway types. Design conventions established many years ago to simplify analyses in the pre-computer era may also oversimplify the analysis of HSOs. For example, SSD is generally analyzed for design purposes along the centerline of the inside lane. Design analyses do not typically develop realistic sightlines taking into account that the driverâs eye is typically located to the left of the vehicle centerline and the vehicle centerline does not necessarily track along the center of the lane. Furthermore, the most critical target that must be seen by a driver to avoid collisionsâanother vehicleâis not a point on the roadway centerline, but an object approximately 6 to 8.5 ft in width and 4.5 to 13.5 ft in height. In addition, the geometry of the horizontal component of SSD (illustrated in Figure 1 for the situation where both the driver and the target to be seen are on the horizontal curve) and its mathematical derivation become more complex than shown in Equation (1) when the driver is on the tangent and the sight obstruction or the target to be seen by the driver is on the horizontal curve, or vice versa (Raymond, 1972). Three-dimensional (3D) computations can construct realistic sightlines for specific design situations to illustrate whether drivers at particular lateral positions in particular lanes can or cannot see specific targets. Reliability analysis provides a mathematical tool to automate such assessments for realistic ranges of key factors. Another concern is that the Green Book assumes a single design situation for SSD, with a vehicle traveling at the design speed, a stationary target to be seen, and controlled braking at a specific deceleration rate. In fact, at any given geometric sight restriction, nearly every aspect of a potential emergency braking situation varies, including vehicle speed, vehicle lateral placement, driverâs eye position within the vehicle, driver behavior, tire/pavement conditions, and target location and dimensions. Visibility restrictions (e.g., heavy rain, fog) may also increase the risk of limited SSD by making it more difficult for approaching drivers to see specific targets, even after they come into view. Reliability analysis can also serve as an effective tool to assess the sensitivity of SSD to these factors, based on their likely variations. The purpose of the reliability analysis would be to assess, at specific horizontal curves, whether these factors are more or less critical.
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. The extent to which a barrier (or retaining wall) is a sight obstruction is a function of the height of the barrier, its offset from the traveled way, and the horizontal and vertical alignment of the roadway. In some scenarios, drivers in the lane closest to the inside of the curve on a roadway or a ramp may have an obstructed view of vehicles ahead; in other scenarios, the driverâs view ahead may be only partially obstructed or not obstructed at all, if the barrier is low and the roadway align- ment allows the driver to see over the barrier. Other potentially challenging scenarios include locations on structures where the bridge rail becomes a sight obstruction; underpasses; and locations with trees, brush, buildings, retaining walls, or other structures close to the road. High-occupancy vehicle (HOV) facilities often have adjacent barriers or retaining walls. An extreme case for the importance of horizontal sightlines occurs on curved roadways in tunnels. The design guidelines presented in this document are based on research by Potts et al. (2018). 1.2 Typical Locations with Horizontal Sight Obstructions This section presents typical locations with horizontal sight obstructions to illustrate the challenges that highway agencies face in making obstruction removal and mitigation decisions. Figure 2 presents two examples of horizontal curves to the right with sight distance limita- tions on rural two-lane highways. In both cases, trees located on the inside of a horizontal (a) (b) Figure 2. Typical curves with horizontal sight obstructions on rural two-lane highways.
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. Figure 3 presents two examples of horizontal curves to the left with sight obstructions on urban mainline freeways. In Figure 3(a), sight distance for an approaching driver is limited by a concrete median barrier. In Figure 3(b), sight distance for an approaching driver is limited by a retaining wall in the highway median. Figure 4 presents a typical rural freeway with a horizontal curve to the left with a concrete median barrier that serves as a sight obstruction. Figure 5 presents four photographs of interchange ramps with limited horizontal sight dis- tance. Figure 5(a) shows trees and a concrete barrier that serve as sight obstructions on a curve to the right near the gore area of a freeway exit ramp. Figure 5(b) shows a curve to left on a ramp where the median barrier between an exit ramp and the entrance ramp in the opposing direction of travel serves as a sight obstruction. Figure 5(c) shows a retaining wall that serves as a sight obstruction on the inside of a curve to the right. Figure 5(d) shows a curve to the right in the downstream portion a ramp for which a bridge abutment and retaining wall on the inside of the curve serve as a horizontal sight obstruction. Figure 3. Typical curves with horizontal sight obstructions on urban mainline freeways. (a) (b) Figure 4. Typical curve with a horizontal sight obstruction on a rural mainline freeway.
8 Design Guidelines for Horizontal Sightline Offsets (a) (b) (c) (d) Figure 5. Typical curves with horizontal sight obstructions on interchange ramps.
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 2âDesign Criteria for HSOs Chapter 3âRelationship of Sight Distance to Crash Frequency and Severity Chapter 4âBenefit-Cost Analysis Chapter 5âReliability Analysis Model for Horizontal Curves with Limited SSD Chapter 6âAssessing Removal or Mitigation of Horizontal Sight Obstructions Chapter 7âDesign Exceptions and Mitigation Strategies References Appendix AâComputation of HSOs Appendix BâUsers Guide for Reliability Analysis Tool Appendix CâCase Studies of Existing Roadways with Sight Obstructions