Chapter 6 - Cross Section Elements

6.2. Typical Section Geometrics

6.2.3. Roadway Slopes

The AASHTO Roadside Design Guide specifies the maximum (steepest) side slope that can be used on a project in order to meet clear zone requirements. Where a range of slopes is given, while slopes as steep as the maximum are allowed, the Designer should strive to provide as flat a slope as feasible.

All front slopes should be 4:1 or flatter. GDOT discourages the use of 2:1 front slopes with guardrail unless economics (construction costs), right-of-way impacts or environmental impacts outweigh the positives of a 4:1 front slope.

GDOT prefers the use of 6:1 front slopes on ditch sections with speed designs equal to or greater than 65 mph, but 4:1 front slopes are allowed as long as clear zone requirements are met.

While the use of a “barn roof” side slope is allowed by GDOT, it is not recommended. A “barn roof” side slope is defined as a 4:1 or flatter front slope extended to the clear zone where a break in slope is provided at a 2:1 side slope rate. From that point, the 2:1 front slope can be used until it intercepts the existing ground or proposed ditch grade. This will help to minimize embankment and right-of-way requirements while eliminating the need for barrier.

In addition to the safety benefits, in urban and residential areas, slopes 4:1 or flatter can be mowed easily with a lawnmower. Efforts to save trees and other items sometimes complicate this procedure, and each residential lot should be addressed separately. Configurations should result in both a pleasing appearance and an easily maintainable configuration.

Refer to Chapter 5. Roadside Safety and Horizontal Clearance of this Manual and Chapter 3 of the current AASHTO Roadside Design Guide, for further discussion about roadway slopes.