Chapter 4 - Elements of Design

4.1. Sight Distance

4.1.5. Intersection Sight Distance  

Intersection sight distance is the sight distance needed for driver decisions at a complex location, such as an intersection. Intersection sight distance values are substantially greater than those of stopping sight distance.

The use of intersection sight distance values less than those specified in the AASHTO Green Book is discouraged and shall require a design variance. In order for a design variance for intersection sight distance to be approved, appropriate mitigation measures (such as no right turn on red or channelization) shall be considered. Refer to the AASHTO Green Book, Chapter 9. Intersections – Sight Triangles for additional information.

Appropriate and applicable intersection sight distance shall be provided for all movements within all intersections.  Appropriate calculations shall be submitted with the Preliminary Field Plan Review (PFPR) materials.  A design variance is required where intersection sight distance is not provided. The following conditions shall be specifically noted:

• minor road alignment in skew

• mainline in horizontal curve

• mainline with crest vertical curve on either side of intersection

• mainline and/or minor road within a cut

These conditions should be checked for both rural and urban sections and are critical for urban sections with narrow shoulders and limited right of way where obstructions on private property may encroach into the sight triangles. Special consideration should be given to obstructions within the right of way, such as: bridges, retaining walls, signs, landscaping, signal control boxes, guardrail, etc.  The AASHTO Green Book (2004), Exhibit 3-54 Diagram Illustrating Components for Determining Horizontal Sight Distance, illustrates and provides further discussion of the affects of obstructions located on the inside of horizontal curves.