Drainage gradient (DG) is a term in road technology, defining the resulting vector of a road surface cross slope (CS) and longitudinal gradient (hilliness).
If the DG is too low, rain and melt water drainage will be insufficient. This results in water pooling on the road surface, thereby increasing the risk for hydroplaning (tires) and vehicle crash.
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Road design manuals worldwide all demand drainage gradient to exceed 0.5 %, in order to drain water and prevent excessive skid accidents[1].
One exception to the minimum 0.5 % DG limit can be found in the Norwegian road design manual, where the limit value for minimum drainage gradient is 2 % instead of 0.5 %[2].
Typically on straight road sections, the drainage gradient is more than 1 - 3 % due to the normal Cross slope (CS) of 1 - 3 %.
In curved sections the drainage gradient is higher, and may often reach 5 - 12 % due to CS in terms of superelevation that may reach 5 - 8 % in areas with icy roads and up to 12 % in areas without icy roads.
Also Grade (slope) contributes to a high drainage gradient. However, longitudinal highway slopes steeper than 0.5 % are surprisingly rare outside mountainous areas.
Due to the normally large CS and the interaction with hilliness, road sections with risk for insufficient drainage gradient are few and short. Still they account for an unacceptable number of skid accidents. These hot spots are found at the entrances and exits of some curves, where the cross slope changes direction in order to create superelevation. In countries with right hand traffic this takes place at left hand curves.
A large study in Sweden has showed that outercurves have 5 times more crashes than innercurves[3]. This finding can partially be explained by the entrances and exits of banked outercurves having insufficient DG[4].
Roads should be designed in such a way that sections where the cross slope change direction (and sign), are located where the road is going uphill or downhill. Otherwise the pavement will get an area with too little drainage gradient (< 0.5 %), resulting in unacceptable skid accident risk.
When designing road curves in a flat landscape, it may be necessary to design long wave undulations on purpose. These "synthetic" longitudinal gradients can then be used to reach a sufficient drainage gradient, in sections where the cross slope is close to zero.
Another option to minimize crash risk due to low DG at entrance or exit of banked outercurves, is to move the superelevation further from the curve and out to a straight road section. This results in a banked straight lane. This design can yield another risk, since the water film (when raining) on an adjacent lane may become thicker. However this is on a straight section where the lack of road curvature minimize the lateral forces and thus keep the skid risk low.
Yet an option is to – within the superelevation transition section – increase the cross slope "tilt rate" within the zone where the cross slope is between -0.5 to + 0.5 %.