How to Use the Superelevation Calculator
A vehicle rounding a horizontal curve is pushed outward by centripetal demand. Banking the pavement toward the inside of the curve offsets part of that demand and side friction between the tire and the pavement carries the rest. This calculator applies the AASHTO point mass relationship to give the superelevation rate a curve needs and the minimum radius allowed at that design speed.
The governing equation is e/100 + f = VĀ²/(15R), with speed in miles per hour and radius in feet. The side friction factor is not a measure of available friction but a comfort threshold, and AASHTO lowers it as speed rises, from about 0.27 at 20 mph down to 0.08 at 80 mph, because drivers tolerate less lateral acceleration at highway speeds.
Maximum superelevation is a policy choice made by each state highway agency. Rates of 4% are typical on curbed urban streets where vehicles stop on the curve, 6% and 8% are the common statewide values, and 10% or 12% appear only on rural roads in regions without significant snow and ice. Where the required rate exceeds the maximum, the radius has to be increased rather than the banking.
When the required rate falls below roughly 1.5%, the curve is usually left at normal crown so that drainage is not compromised. Full design also has to cover the superelevation runoff and tangent runout lengths, the axis of rotation, and spiral transition lengths, so treat this result as a planning level check of the curve itself.
Frequently Asked Questions
The banking cannot simply be increased past the agency maximum, so the radius has to grow. Use the minimum radius shown for that speed and emax as the flattest curve permitted, or lower the design speed for that section.
On ice, available friction drops sharply and a stopped or slow moving vehicle can slide toward the inside of a steeply banked curve. Agencies in those regions therefore cap superelevation at 6% or less, and curbed urban streets are limited for the same reason.