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Superelevation is the amount of cross slope needed in a roadway that will allow the vehicle to safely traverse around a curve without slipping at a given design speed. It maybe abbreviated as S.E. or just as SE according to some reference manuals. SE is expressed in a percent or a vertical distance drop per foot (or meter). An 8% SE would represent a rate of slope equivelant to 0.08 feet (meter) drop per foot (meter) traveled. Notations such as 8% or 0.08' / ' are common and both are considered acceptable by many organizations.
The sloping of the road provides a smoother rider for the motorist and allows the driver to enter the curve at a higher rate of speed. An example of SE that is easy to see is the banking used in auto racing. These curves will most often have a high bank in the curve where the outside portion of the curve is higher than the inside portion of the curve. These 'high banks' are a form of SE. that allows the driver to enter the curve at a higher rate of speed and keeps the driver from slipping sideways as the curve is traveled.
SE reduces the amount of centripetal acceleration that is placed upon the driver at the apex of the curve. Without superelevation in highways, the driver would tend to be thrown to one side of the vehicle as the curve is traversed. The driver would also need to slow down to take the same raidus of curve without SE.
For highway design, superelevation rates range from flat to upwards of 12% or more. Superelevation rates of 4% are common in urban areas while rural high speed highway use 8%. Local governments will most often dictate maximum rates based upon the classification of the road. In northern climates where snow and ice is common, lower superelevation rates are chosen because of the tendency of the vehicle to slide across the pavement during frozen precipitation.
In general, superelevation rates are based upon the degree of curvature of the road and the speed design of the road. The smaller the radius of the curve, the higher the superelevation rate that is used by the designer. Roads with a high speed design will require a higher rate of superelevation. Inversely, roads with a low speed design will require a lower rate of SE. Curves with a large radius or low speed design may not require any superelevation. Most often, residential roads that have a low speed design will not be superelevated.
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