Risk assessment of SSD offers insights into mitigating a location with limited SSD. Increasing sight distance through the reconstruction (lengthening) of a vertical curve is typically considered first. This measure, however, is often expensive and may result in adverse impacts to access or adjacent rights-of-way.

Risk assessment of SSD offers insights into mitigating a location with limited SSD. Increasing sight distance through the reconstruction (lengthening) of a vertical curve is typically considered first. This measure, however, is often expensive and may result in adverse impacts to access or adjacent rights-of-way. Other effective mitigation measures may include the relocation or removal of other features located within the sight-restricted alignment, spot widening the roadway or shoulder to increase space for crash avoidance, and signing, lighting, and delineation treatments. Note that for severe sight restrictions, improvements can be effective even if minimum SSD is not provided but some increase is gained. Where a project involves the reconstruction of an existing highway, which includes locations with limited vertical curvature and thus less than desirable SSD, designers should study the known crash history of the road and the locations to determine the extent of actual safety risk. Research and experience suggest that marginally deficient SSD may not translate into actual safety problems. The analysis should also consider the character of the roadway at the location of poor SSD. Referring to the above guidance, it may often be found that retention of the alignment that is just outside the traditional range is acceptable and more cost-effective than reconstruction to full criteria.

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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