Design Guidelines for Horizontal Sightline Offsets (2019) / Chapter Skim
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Pages 49-56
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From page 49... ...
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)
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From page 50... ...
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.
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From page 51... ...
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.
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From page 52... ...
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.
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From page 53... ...
Design Exceptions and Mitigation Strategies 53 Figure 18. Typical advance warning sign for a horizontal curve.
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From page 54... ...
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.
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From page 55... ...
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.
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From page 56... ...
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.
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