Chapter 4 - Elements of Design

4.1. Sight Distance

4.1.1. General Considerations

Sight distances for all projects shall be checked. A detailed explanation of how to apply the sight distances is described in the American Association of State Highway and Transportation Officials (AASHTO) publication, A Policy on the Geometric Design of Highways and Streets (Green Book), Chapter 3, Design Elements.  In addition, Chapter 9 of the Green Book (Intersections) discusses special conditions related to sight distance at intersections. 

General considerations relating to sight distance noted in the AASHTO Green Book include:

• Safe and efficient operation of a vehicle is highly dependent on adequate sight distance.
• Two-lane rural highways should generally provide sufficient passing sight distance at frequent intervals and for substantial distances.  Conversely, passing sight distance on two-lane urban streets/arterials is normally of little value.
• The proportion of a highway’s length with sufficient sight distance to pass another vehicle and interval between passing opportunities should be compatible with design criteria pertaining to functional classification, as discussed in this Manual in Chapter 2. Design Policies and Standards.
• Special consideration should be given to the sight distance requirements at queue backups over a hill, signals, horizontal curves, turning movements, barriers, guardrails, structures, trees, landscaping, vegetation and other special circumstances.