Transportation

Curve Superelevation

Curve superelevation calculator — required superelevation rate, minimum radius, and runoff length for horizontal curves per the AASHTO e+f method.

Transportation · AASHTO

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.

AASHTO Superelevation Design Equations

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]

Reference: AASHTO Green Book 2018, Chapter 3; Tables 3-7 (f_s,max by speed), 3-12 (Method 5 distribution), and 3-15 (max relative gradient Δ). e in %, V in mph, R in ft.

Superelevation Calculator

Imperial units — mph, feet, percent

Design Speed (mph)

Curve Radius R (ft)

Maximum e_max (%)

Lanes Rotated (N_L)

Lane Width w (ft)

Superelevation Results

Required Superelevation (e_req)—

Design Status—

Centripetal Demand (V²/15R)—

Side Friction Used (f_s)—

Minimum Radius for e_max—

Superelevation Runoff (L_r)—

Tangent Runout (L_t)—

Max Relative Gradient (Δ)—

Rotation about centerline (Method 5 per AASHTO Table 3-12). Verify spiral/transition requirements and adjust L_r for divided and multilane highways. Normal crown assumed at 2%. Requires PE review.

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