Where limited ISD exists according to the AASHTO Green Book (2), the designer has a number of options depending on the nature of the sight restriction. Every attempt should be made to eliminate sight restrictions such as trees, vegetation, signs, and movable obstacles. Where such obstacles cannot be moved, field investigations can determine whether the driver can safely position the vehicle such that actual sight lines (versus those specified in the design model) are clear.

Where limited ISD exists according to the AASHTO Green Book (2), the designer has a number of options depending on the nature of the sight restriction. Every attempt should be made to eliminate sight restrictions such as trees, vegetation, signs, and movable obstacles. Where such obstacles cannot be moved, field investigations can determine whether the driver can safely position the vehicle such that actual sight lines (versus those specified in the design model) are clear.

Sight restrictions associated with vertical geometry require geometric revisions. These may include alignment reconstruction, but also may include the relocation of the intersection away from the sight restriction, closure of the intersection, or turn restrictions that eliminate higher risk movements. In urban areas, the creative use of turn restrictions, focusing traffic onto safer (perhaps signal-controlled) intersections, is a viable solution. Advance signing on the unstopped road to warn of the intersection can also be used. Finally, where volumes are high, sight restrictions significant, and a pattern of crashes related to the sight restriction is evident, traffic signal control may be the appropriate solution. Of course, visibility of the signal is important on all approaches, particularly along highways where traffic is unstopped for long distances and the presence of the signal may be unexpected.

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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