Where acceptance of nominally sharper-than-normal curvature appears to be an appropriate solution, the designer has many tools available to mitigate the potential adverse safety impacts. Widening the road and/or shoulder through the curve and improving the roadside may address crash severity. Relocating or closing intersections or driveways within the curve removes additional risk factors. Spot resurfacing or wedging of the pavement through the curve can increase available friction for cornering. Mitigation measures for horizontal curvature below typical ranges may include one or more of the following:

Where acceptance of nominally sharper-than-normal curvature appears to be an appropriate solution, the designer has many tools available to mitigate the potential adverse safety impacts. Widening the road and/or shoulder through the curve and improving the roadside may address crash severity. Relocating or closing intersections or driveways within the curve removes additional risk factors. Spot resurfacing or wedging of the pavement through the curve can increase available friction for cornering. Mitigation measures for horizontal curvature below typical ranges may include one or more of the following:

• Signing
• Delineation
• Increased superelevation
• Lane widening
• Shoulder paving
• Increased clear zone

The importance of providing a forgiving roadside generally increases as the alignment becomes more severe due to the increased likelihood of errant vehicles. Greater emphasis should be placed on roadside safety when atypical alignment elements are used to reduce the cost and impacts of the project. Indeed, the decision to accept the use of atypical alignment may in part be based on the ability to compensate through special roadside design treatments.

Designers may also strive to reduce vehicle speeds on the approaches to the curve through other alignment revisions. The use of transition curves to step down operating speeds may be preferable to abrupt changes in alignments that violate driver expectancy. The use of spiral transitions has been shown to positively influence the safety and operations of some curves. Increasing superelevation through the curve may also be considered. While increased superelevation can help reduce crash rates for sharp horizontal curves, designers need to address potential adverse design impacts on the roadside, driveways, cross streets, and adjacent properties.

Finally, adequate justification should be provided and documented when the alignment will not conform to the design values in the AASHTO Green Book (2) or state guidelines.

From A Guide for Achieving Flexibility in Highway Design, 2004, by the American Association of State Highway and Transportation Officials, Washington, D.C. Used by permission. AASHTO publications may be purchased from that organization's bookstore at 1-800-231-3475 or online at http://bookstore.transportation.org.

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