4.10.4 Pavement Cross Slope
The operating characteristics of vehicles on crowned pavements are such that for cross slopes up to 2 percent, the effect on steering is barely perceptible. A reasonably steep lateral slope is desirable to minimize water ponding on flat sections of uncurbed pavements due to imperfections or unequal settlement. With curbed pavements, a sufficiently steep cross slope is desirable to contain the flow of water adjacent to the curb.
The recommended pavement cross slope for usual conditions is 2 percent.
Pavement cross slopes on all roadways should not be less than 1 percent.
In areas of intense rainfall, steeper cross slopes may be used as discussed in
.
Highways with three or more lanes inclined in the same direction should desirably have an increasing cross slope as the distance from the crown line increases to facilitate pavement drainage. In these cases, the first two lanes adjacent to the crown line may be sloped flatter than normal-typically at 1.5 percent but not less than 1 percent. The cross slope of each successive pair of lanes (or single lane if that is the outside lane) outward from the crown should be increased by 0.5 to 1.0 percent from the cross slope of the adjacent lane.
A cross slope should not normally exceed 3 percent on a tangent alignment unless there are three or more lanes in one direction of travel.
Cross-slopes greater than 2 percent should be limited to use in areas of intense rainfall.
In areas of intense rainfall and where three or more lanes are provided in one direction of travel, the maximum cross slope within the tangent section of an alignment should be limited to 4 percent.
Bridge structures with three or more lanes in one direction, should maintain at least a constant slope of 2.5 percent, transitioning before and after the bridge accordingly
For tangent sections on divided highways, each pavement should have a uniform cross slope with the high point at the edge nearest the median. Although a uniform cross slope is preferable, on rural sections with a wide median, the high point of the crown is sometimes placed at the centerline of the pavement with cross slopes from 1.5 to 2 percent. At intersections, interchange ramps or in unusual situations, the high point of the crown position may vary depending upon drainage or other controls
For two-lane roadways, cross slope should also be adequate to provide proper drainage.
The cross slope for two-lane roadways for usual conditions is 2 percent and should not be less than 1 percent.
Shoulders should be sloped sufficiently to drain surface water but not to the extent that safety concerns are created for vehicular use.
The algebraic difference of cross slope between the traveled way and shoulder grades should not exceed 6 percent. Maximum shoulder slope should not exceed 10 percent.
The following are recommended cross slopes for various types of shoulders:
- Bituminous and concrete-surface shoulders on tangents should be sloped from 2 to 6 percent. Often the slope rate is identical to that used on the travel lanes, for constructability, smooth transition, and ease of use during construction and maintenance traffic control;
- Gravel or crushed rock shoulders should be sloped from 4 to 6 percent; and
- Turf shoulders should be sloped at about 8 percent.
4.10.4.1 Evaculanes
For hurricane shoulder evacuation lanes (Evaculanes) and for facilities where widening is anticipated to accommodate the ultimate typical section (i.e., using the proposed shoulders as future traffic lanes) the shoulder cross-slope should be designed in accordance with the criteria for traffic lanes. See
for additional information.