Work out superelevation rate instantly with clear inputs, formula shown and shareable results.
Balancing centrifugal force on a curve gives e + f = V² / (127 R) with speed in kilometres per hour and radius in metres. Subtracting the available side friction leaves the superelevation the road must provide. Where the result exceeds the maximum permitted crossfall, either the radius must increase or the speed must be restricted.
Superelevation
e + f = V² / (127 R), so e = V² / (127 R) - f
Limits
Adopted e is capped at the maximum permitted crossfall, commonly 7 percent
Preliminary estimate only. Superelevation, side friction values and run-off design must follow the geometric design standard adopted by the road authority.
e + f = V² / (127 R), so e = V² / (127 R) - f. Balancing centrifugal force on a curve gives e + f = V² / (127 R) with speed in kilometres per hour and radius in metres.
A steeply banked curve is dangerous for slow or stationary vehicles, which can slide toward the inside, and it is uncomfortable in wet or icy conditions. Seven percent is the usual limit on roads with mixed traffic.
This calculator takes 4 inputs: Design speed, Curve radius, Coefficient of side friction, Maximum permitted superelevation. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.