Superelevation, also known as cant, is a design feature used in roadway curves where the outer edge of the pavement is raised above the inner edge. It is the rate of change in elevation (height) between the two edges of the road or rails). The purpose of superelevation is to counteract the effect of centrifugal force and reduce the tendency of vehicles to skid laterally outwards by raising the pavement outer edge with respect to the inner edge. Superelevation is expressed as a decimal, representing the ratio of the pavement slope to width, ranging from 0 to 0.12 foot/feet. The key features of superelevation include:
- Maximum superelevation rate: The highest percentage of angle allowed to prevent the danger of vehicles overturning. Expressed in formulas as e max.
- Minimum superelevation: The lowest percentage of angle allowable for road drainage purposes, especially on a large-radius horizontal curve. Typically 2% to 4%.
- Slope: A measurement of steepness, level of incline, or upward or downward slant, as in pavement slope.
- Spiral runoff: A section of highway that transitions the angle and width between a section of tangent runout and a section of full superelevation, making it easier for drivers to navigate the change in curvature and angle.
- Superelevation rate: The degree of banking imposed on a horizontal curve to safely counterbalance the centrifugal force of a vehicle on the curve.
Superelevation is a crucial element in roadway design, and without it, curves have to be taken much more slowly for safety.
