Chapter 4 - Elements of Design

4.1. Sight Distance

4.1.2. Stopping Sight Distance

Stopping sight distance, which is the sight distance needed for bringing a vehicle to a stop, is required at all locations on all roadways.

The use of stopping sight distance values less than the desirable values specified in the AASHTO Green Book is strongly discouraged and shall require a design exception.

Stopping sight distance across the inside of curves plays a critical role in determining roadway horizontal curvature and applicable shoulder widths.  There shall be no obstruction of sight distance on the inside of curves (such as median barriers, walls, cut slopes, buildings, landscaping materials, and longitudinal barriers). If removal of the obstruction is impractical to provide adequate sight distance, a design may require adjustment in the normal highway cross section or a change in the alignment.

Because of the many variables in alignment, cross section, and in the number, type, and location of potential obstructions, the actual conditions on each curve should be checked and appropriate adjustments made to provide adequate sight distance. The AASHTO Green Book (2004), Exhibit 3-54 Diagram Illustrating Components for Determining Horizontal Sight Distance, provides additional information on the effects of obstructions located on the inside of horizontal curves.