Superelevation Rate Calculator

Work out superelevation rate instantly with clear inputs, formula shown and shareable results.

About Superelevation Rate Calculator

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.

Formulas

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

Worked example

With the default values below, Superelevation Rate Calculator returns Superelevation required of 1.798%. Change any input above and every figure here is recalculated in your browser.

Inputs used

Design speedminimum 10 km/h: 80 km/h
Curve radiusminimum 5 m: 300 m
Coefficient of side frictionaccepts 0.01–0.4: 0.15
Maximum permitted superelevationaccepts 1–12 %: 7 %

Results

Superelevation required: 1.798% %
Superelevation ignoring friction: 16.798% %
Superelevation to adopt: 1.80% %
Assessment: Superelevation within the permitted maximum

Step by step

1Values used
2Superelevation
3Limits
4Superelevation required= 1.798 %
5Superelevation ignoring friction= 16.798 %
6Superelevation to adopt= 1.80 %
7Assessment= Superelevation within the permitted maximum

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, fees or charges or compounding frequency into account.
Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
Preliminary estimate only. Superelevation, side friction values and run-off design must follow the geometric design standard adopted by the road authority.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Preliminary estimate only. Superelevation, side friction values and run-off design must follow the geometric design standard adopted by the road authority.

Frequently Asked Questions

How is the Superelevation Rate figure worked out?

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.

Why cap superelevation?

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.

What values do I need to enter?

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.