Drainage gradient

This results in water pooling on the road surface, thereby increasing the risk for hydroplaning and wet-pavement vehicle crashes.

Most road design manuals require drainage gradient to exceed 0.5%, in order to drain water and prevent excessive skid accidents.

Due to the normal cross slope and the interaction with grade, road sections with insufficient drainage gradient are few and short.

These hot spots are found at the entrances and exits of banked curves, where the cross slope changes direction in order to create superelevation.

As the outside edge of the curve is raised (or superelevated) to create the bank, it passes through a point where the cross slope is absolutely flat.

Another option to minimize crash risk due to low DG at the entrance or exit of banked outercurves is to move the superelevation further from the curve and out to a straight road section.