Chapter 4 - Elements of Design

4.4. Combined Horizontal and Vertical Alignments

4.4.2. Safety Considerations

The superimposed effect of horizontal and vertical alignments can influence both sight distance and driver expectancy – which translate directly into safety. As safety should be the designer’s primary consideration, the following guidelines are presented:

• Sharp horizontal curves should not be introduced at or near the top of a pronounced vertical curve, since the driver cannot perceive the horizontal change in alignment, especially at night.

• Sharp horizontal curves should not be introduced at or near the low point of a sag vertical curve, since vehicles, particularly trucks, are traveling faster at the bottom of grades.

• Both horizontal and vertical curvature should be as flat as possible at intersections where vehicles have to decelerate, stop, or accelerate.

• To maintain proper pavement drainage, design vertical and horizontal curves so that the flat profile of a vertical curve will not be located near the flat cross slope of the superelevation transition. As a general rule, pavement cross slope should be at least 1.0% near vertical curve sag points and longitudinal roadway grades should be at least 0.30% at locations where the pavement cross slope is flat (0%), for instance at superelevation transitions.

• On two-lane roadways, the need for safe passing sections (at frequent intervals and for an appreciable percentage of the length of the roadway) often supersedes the general desirability for combination of horizontal and vertical alignment. The Designer should strive to implement long tangent sections to secure sufficient passing sight distance.

• It is generally poor practice to place the superelevation rotation point at a different point than the profile grade line.

• Particular attention shall be paid to all forms of sight distance when horizontal and vertical alignments are superimposed on each other. The combination of horizontal and vertical curvature can sometimes result in effectively less sight distance than the individual affect of either horizontal or vertical curvature.