Superelevation

Superelevation (cant) is the transverse tilting of a road/rail carriageway on a horizontal curve, raising the outer edge above the inner, so that a component of the vehicle's weight helps counteract the outward centrifugal force and keeps cornering safe and comfortable. The required value follows from the equilibrium relation e + f = V²/127R (e = superelevation rate, f = lateral friction factor, V in km/h, R = radius in m), balanced so that neither e nor f alone is overstressed.

Indian highway practice (IRC 38 design tables for horizontal curves, IRC 73 geometric design of rural highways) caps the maximum superelevation (typically 7%, lower in snowbound or urban/built-up areas) and limits the rate of attainment so the transition is comfortable; the superelevation is introduced gradually over the transition (spiral) curve, with the pavement rotated about the centre-line or an edge as specified. Correct superelevation, the curve radius and the transition together govern safe design speed; deficient or badly attained superelevation is a recognised cause of run-off-road and skidding crashes.