Provision for sufficient ISD is recognized in the AASHTO Green Book (2) as important for overall intersection operations. For unstopped approaches, the Green Book (2) notes that the provision of SSD will generally provide sufficient distance for drivers to perceive conflicts. Intersection sight distances that exceed stopping sight distances for unstopped vehicles "enhance traffic operations." Provision for sufficient ISD is recognized in the AASHTO Green Book (2) as important for overall intersection operations. For unstopped approaches, the Green Book (2) notes that the provision of SSD will generally provide sufficient distance for drivers to perceive conflicts. Intersection sight distances that exceed stopping sight distances for unstopped vehicles "enhance traffic operations."

Designers are afforded flexibility in the key parameters for ISD design, including, most importantly, selection of the approach design speed. Designers also are expected to use judgment as to which design vehicle should be the basis for ISD computations: a passenger car or larger vehicle.

The AASHTO Green Book (2) also recognizes that intersections designed to operate with certain types of intersection control (signal, all-stop) require less stringent sight lines.

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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