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49 This chapter explains the situations in which design exceptions for horizontal sight distance should be prepared and the mitigation strategies that may be considered where a horizontal sight obstruction is not removed. 7.1 Design Exceptions FHWA has established 10 controlling criteria for geometric design that must be met on projects on the National Highway System (NHS) or a design exception document must be prepared. Two of the 10 controlling criteria, structural loading capacity and design speed, apply to all NHS roads and streets. The remaining eight controlling criteria, including SSD, apply only to NHS roads that are Interstate highways, other freeways, and roads with design speeds of 50 mph or greater. Any project on these facility types containing a horizontal curve for which the ASSD is less than the applicable value of DSSD for which the sight restriction is to remain in place would require a design exception to be prepared and approved. Inter- nal policies of individual highway agencies may require design exceptions for other design situations, as well. A design exception document typically describes the project site, the geo- metric design element that does not meet design criteria and is to remain in place, the traffic operational and safety performance of the site, and the mitigation strategies (if any) that will be implemented. 7.2 Potential Mitigation Strategies for Limited Horizontal Sight Distance Where horizontal sight obstructions are identified on the inside of horizontal curves, their removal should be considered. A highway agency may conclude that it is impractical to remove some horizontal sight obstructions where the cost to do so would be high. In such cases potential mitigation strategies should be considered. The costs of the mitigation strategies themselves range from low to very high, and their practicality also needs to be assessed. Table 4 summarizes the potential mitigation strategies that may be considered by highway agencies once a decision has been reached that it is impractical to remove a sight obstruction on the inside of a specific horizontal curve. The table identifies each potential mitigation strategy and presents the location type(s) to which the strategy is applicable, the relative mitigation cost for new construction projects and projects on existing roads, and comments on implementa- tion of the strategy. The mitigation strategies presented here were identified from the FHWA publication, Mitigation Strategies for Design Exceptions (Stein and Neuman 2007), and from the research by Potts et al. (2018). C H A P T E R 7 Design Exceptions and Mitigation Strategies
50 Design Guidelines for Horizontal Sightline Offsets Mitigation strategy Applicablelocation type(s) Relative mitigation cost CommentsNew construction projects Projects on existing roads Remove a portion of the sight obstruction All Low to very high Low to very high Cost is potentially lower than removing the entire sight obstruction. Reduce the height of the sight obstruction All Low to very high Low to very high An example would be using a concrete barrier with reduced height. Widen inside shoulder adjacent to sight obstruction All Medium to high Medium to very high For projects on existing roads, this strategy may involve narrowing the travel lanes or realigning the entire roadway. Widen inside shoulder in the sight- restricted area All Medium to high Medium to very high Provides an improved primary recovery area on the portion of the roadway where the driver may encounter an unexpected condition. Switch inside and outside shoulder widths so that the wider shoulder is on the inside of the horizontal curve Ramps Low Low to very high Very little cost at the design stage in new construction; on existing roads, involves restriping only if the shoulders have been designed to accommodate traffic loads. Realign roadway All Medium to very high High to very high Realignment of the roadway may cost as much or more than removing the sight obstruction. Widen median Divided highways High to very high High to very high Widening a median may make it possible to remove a barrier or locate the barrier further from the traveled way, but will likely also involve realignment of one or both roadways. Install advance curve warning signs All Low Low Particularly desirable where the presence of the curve is not apparent to approaching drivers. Install advance curve warning signs with advisory speed plates All Low Low Desirable where drivers need to reduce speed on particular curves. Install advance warning signs for geometric features in the sight- restricted area All Low Low Desirable where conflict or decision points, such as intersections, driveways, ramp junctions, or a subsequent curve is located in the sight-restricted area. Relocate existing signs to better positions All N/A Low to medium Locate signs in highly visible positions, sufficiently far in advance of the horizontal curve or sight-restricted area. Install dynamic signing with flashing beacons triggered by vehicles exceeding the speed limit or advisory speed All Low to medium Low to medium May be appropriate at mainline locations and isolated ramp curves; not desirable for curves adjacent to decision points, such as the diverge point on an off-ramp. Improve delineation All Low Low Improve the visibility of the horizontal curve to approaching drivers by improving centerline, lane line, and edgeline markings on the curve or installing chevron warnings on the outside of the curve. Remove conflict or decision points from sight-restricted area All Medium to high Medium to very high Costs to remove intersections, driveways, off-ramps, or subsequent curves from the sight-restricted area may range from medium (for driveways) to high or very high (for other features). Provide lighting for intersections or other features in the sight-restricted area All Medium to high Medium to high Lighting may assist drivers in identifying potential conflicts at intersections or decision points to which sight distance may be limited. Provide real-time warning of crashes or queues ahead Urban freeways Medium to high Medium to high Real-time warnings are likely to be practical only on managed freeways. Table 4. Summary of potential mitigation strategies for horizontal sight obstructions (Potts et al. 2018).
Design Exceptions and Mitigation Strategies 51 The relative costs for implementation of mitigation strategies range from low to very high, and the cost for any given strategy can vary widely based on site conditions. Mitigation costs are likely to be low for strategies that involve signing or other traffic control devices, but can be high to very high for strategies at sites where implementation of the strategy would involve structures, earthwork, roadway reconstruction, or right-of-way acquisition. There are no direct CMFs or measures of effectiveness for mitigation of limited horizontal sight distance. Research in NCHRP Report 783 (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. This general principle may apply to other situations, such as horizontal sight distance limitations, but no quantitative results can be derived because NCHRP Report 783 addressed only crest vertical curves. Each of the mitigation strategies is discussed in the following sections. The previous section discusses the decision-making process for implementing mitigation strategies. 7.2.1 Remove a Portion of the Sight Obstruction Even when it is impractical to remove all of the horizontal sight obstruction, it may be practical to remove a portion of the sight obstruction. For example, where trees and a concrete barrier create sight obstructions, it may substantially improve the available sight distance if the trees were removed, even with the concrete barrier being retained. This would avoid the high cost of realignment and retain the advantages of the barrier, which may be needed as a roadside safety device for other reasons. 7.2.2 Reduce the Height of the Sight Obstruction It may be feasible to reduce the height of a roadside sight obstruction even where it would be undesirable to remove it completely. For example, while many concrete traffic barriers are 4 to 4.5 ft in height, there are approved concrete barriers that are 2.4 to 2.7 ft in height which may be suitable in some locations (Stein and Neuman 2007). 7.2.3 Widen Inside Shoulder Adjacent to Sight Obstruction At some locations, it may be possible to increase horizontal sight distance by increasing the distance between the sight obstruction and the inside edge of the traveled way by widening the inside shoulder of the roadway. This would typically involve either narrowing the travel lanes or realigning the roadway. 7.2.4 Widen Inside Shoulder in the Sight-Restricted Area Where it is not feasible to widen the inside shoulder adjacent to the sight obstruction, it may be feasible to widen the inside shoulder downstream of the sight obstruction within the sight- restricted area. This would provide an improved primary recovery area for drivers trying to avoid a collision in the sight-restricted area. 7.2.5 Switch Inside and Outside Shoulder Widths The AASHTO Green Book recommends that one-way interchange ramps should have 2 to 4 ft paved shoulders on the left side of the ramp and 8 to 10 ft paved shoulders on the
52 Design Guidelines for Horizontal Sightline Offsets right of the ramp, with a total combined shoulder width of 10 to 12 ft. The unequal shoulder width provides an opportunity, where a ramp has a horizontal sight obstruction on the inside of a curve to the left, to switch the left and right shoulder widths so that the wider shoulder is on the left side of the ramp and the narrower shoulder is on the right side of the ramp. This moves the vehicles further from the inside of the curve and increases the available sight distance on the ramp. This strategy can be implemented at low cost in the design stage of a newly constructed ramp. For an existing ramp, the only implementation cost may involve restriping the edgelines, as long as the paved area onto which traffic is moved is suitable for traffic loads. This strategy is particularly desirable for ramps on structures where any other method of increasing the available sight distance may have a very high cost. 7.2.6 Realign Roadway Realigning roadway(s) to move traffic further from a horizontal sight obstruction and thereby increase sight distance is typically very expensive. If a horizontal sight obstruction that would be impractical to move is recognized early in the design process for a new construction project, the increased cost of realigning a roadway may, in some cases, be modest, but in most situations realigning a roadway is likely to involve high to very high cost. Roadway realignment may cost as much, or more, than removing the sight obstruction would cost. 7.2.7 Widen Median For many curves to the left on divided highways, a continuous median barrier may consti- tute a horizontal sight obstruction. The only feasible method to eliminate the barrier as a sight obstruction may be to widen the median sufficiently that the barrier can be moved further from the traveled way or sufficiently that the barrier is no longer needed. Widening the median may involve very high cost because one or both roadways may need to be realigned and additional right-of-way may need to be acquired. 7.2.8 Install Advance Curve Warning Signs Installation of an advance curve warning sign is desirable in advance of any horizontal curve with limited sight distance. Advance curve warning signs are intended primarily for use where the presence of the curve is not apparent to approaching drivers. However, if drivers are alerted to the presence of the curve by the advance warning sign, they may be better prepared to deal with any potential conflicts they encounter in a sight-restricted area at or beyond the curve. Figure 18 shows a typical advance warning sign for a horizontal curve. At some sites, Pennsylvania has supplemented advance curve warning signs with pavement markings in the travel lane showing the word SLOW and a curve arrow to the left or right, as appropriate. 7.2.9 Install Advance Curve Warning Signs with Advisory Speed Plates Advance curve warning signs may be supplemented with advisory speed plates indicating the appropriate speed at which drivers should traverse the curve. Advisory speeds are typically chosen based on the side friction demands in traversing the curve, which can be measured with a ball-bank indicator. However, the ASSD for the curve may also be useful in determining an appropriate advisory speed. Figure 19 shows two examples of typical advance warning signs for horizontal curves with advisory speed plates.
Design Exceptions and Mitigation Strategies 53 Figure 18. Typical advance warning sign for a horizontal curve. Figure 19. Typical advance warning signs for horizontal curves with advisory speed plates.
54 Design Guidelines for Horizontal Sightline Offsets 7.2.10 Install Advance Warning Signs for Geometric Features in the Sight-Restricted Area Where horizontal sight obstructions are present on a curve, it is highly desirable to provide signing to warn drivers in advance of any conflict or decision points that may be present in the sight-restricted area, including intersections, driveways, pedestrian crossings, ramp junctions, and subsequent horizontal curves. Figure 20 shows typical advance warning signs for intersections and driveways located in sight-restricted areas. 7.2.11 Relocate Existing Signs to Better Positions Warning signs provided in advance of the horizontal curve, whether warning of the curve itself or of other conflict or decision points, should be placed, following criteria in the Manual on Uniform Traffic Control Devices for Streets and Highways (FHWA 2009), in a position that will be visible to the driver and sufficiently far in advance of the feature addressed on the warning sign. It is also desirable to avoid sign clutter, placing warning signs where they are not immediately adjacent to other signs and other demands on the driverâs attention. 7.2.12 Install Dynamic Signing With Flashing Beacons Dynamic signing with flashing beacons triggered by vehicles exceeding the speed limit or the applicable advisory speed are desirable to provide a highly visible warning to drivers. Their use in advance of horizontal curves with limited sight distance should be considered in main- line locations on freeways and other highways or in isolated locations on ramps. The use of dynamic signing may not be desirable on curves located at decision points, such as adjacent to Figure 20. Typical advance warning signs for intersections and driveways located in sight-restricted areas.
Design Exceptions and Mitigation Strategies 55 an off-ramp gore area; dynamic signs addressing one decision may not be desirable at a location where multiple decisions must be made by the driver. 7.2.13 Improve Delineation Delineation may be used to improve visibility of a horizontal curve to approaching drivers. Appropriate delineation includes improved centerline, lane, and edgeline markings, and placement of chevron markers on the outside of the curve. Improved delineation will not inform drivers of the sight restriction, but may assist drivers in choosing an appropriate speed for the curve geo- metrics. Figure 21 shows a typical horizontal curve with chevron markers for added delineation. 7.2.14 Remove Conflict or Decision Point in the Sight-Restricted Area Conflict or decision points, such as intersections, driveways, ramp junctions, and subsequent curves are undesirable with the sight-restricted area downstream of a horizontal curve with a horizontal sight obstruction. Removal of such conflict or decision points is desirable, but may be impractical because of high cost. 7.2.15 Provide Lighting at Intersections and Other Features in the Sight-Restricted Area Lighting of intersections or other features in the sight-restricted area may assist drivers in choosing an appropriate path and speed through those features at night, even with the presence of the sight restriction. Figure 21. Typical horizontal curve with chevron markers for added delineation.
56 Design Guidelines for Horizontal Sightline Offsets 7.2.16 Provide Real-Time Warning of Crashes or Queues Ahead Real-time warnings to drivers of crashes or queues ahead, particularly crashes or queues within a sight-restricted area, are desirable. However, providing real-time warnings may only be practical on managed facilities, such as urban freeways, where the infrastructure to provide such warning messages is already in place.
