Drainage gradient

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Drainage gradient is a term in road technology, defining the resulting vector of a road surface cross slope (CS) and longitudinal gradient (hilliness). Road design manuals worldwide all demand drainage gradient to exceed 0.5 %, in order to drain water and prevent excessive skid accidents. Typically on straight sections, the drainage gradient is more than 1 - 3 % due to cross slope. In curved sections the drainage gradient is higher, and may reach 5 - 9 % due to CS in terms of superelevation. Also hilliness contribute to a high drainage gradient. Thereby risk sections 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 the UK (with left hand traffic) this takes place at right hand curves, but in most European countries (with right hand traffic) this takes place at left hand curves. Roads must 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. As described above, these hot spot sections are located at the entrances and exits of some curves; in the UK right hand curves.