In terms of the 13 controlling criteria, vertical alignment includes grade as well as vertical curvature (both crest and sag); grade considered separately and discussed below. Vertical curvature influences another primary controlling criterion, stopping sight distance. The geometric design basis for minimum length of crest vertical curvature is to provide the minimum stopping sight distance for the combination of grades and design speed. Sag vertical curves are normally designed so the curve does not restrict the distance of roadway illuminated by vehicle headlights, which would reduce stopping sight distance at night. The influence of and design considerations regarding design exceptions for vertical curvature are discussed below in the section on stopping sight distance.
Refer to the sections on grade and stopping sight distance for more information on vertical alignment.
Vertical Alignment Resources
- A Policy on Design Standards Interstate System, AASHTO, 2005.
- A Policy on Geometric Design of Highways and Streets, AASHTO, 2004.
- Guidelines for Geometric Design of Very Low-Volume Local Roads (ADT ≤ 400), AASHTO, 2001.