Transportation
Curve Superelevation
Curve superelevation calculator — required superelevation rate, minimum radius, and runoff length for horizontal curves per the AASHTO e+f method.
Curve Superelevation
Superelevation is the transverse inward banking applied to a roadway on a horizontal curve to counteract centrifugal force. AASHTO balances superelevation (e) and lateral friction (f) so their sum equals the centripetal acceleration demand V²/(15R). Both e and f are consumed at the same rate as radius decreases.
The maximum superelevation e_max is set by context: 4–6% for urban or icy conditions, 8% for rural, 10–12% for high-speed limited-access facilities in warmer climates. The superelevation runoff length L_r defines the transition over which the outside lane rotates from normal crown to full e — governed by AASHTO maximum relative gradient limits by speed.
e + f = V² / (15 · R) [V in mph, R in ft]
R_min = V² / [15 · (f_max + e_max/100)]
L_r = (w · N_L · e_req) / Δ_max [ft]
At 50 mph on a 1,200-ft radius curve, the centripetal demand V²/15R works out to 13.9%. AASHTO's side friction factor at 50 mph is f_s = 0.19 — already higher than the demand — so the required superelevation e_req comes out negative, meaning a normal 2% crown is adequate and no additional banking is needed. The minimum radius that would require the full 6% e_max at this speed is 667 ft, well below the 1,200-ft radius used here.
Superelevation Calculator
Imperial units — mph, feet, percent