What is superelevation?

Superelevation is the transverse slope (banking) applied to a road surface on a horizontal curve. It counteracts the centrifugal force on a vehicle, reducing the side friction needed to keep the vehicle on the road.

What formula does this calculator use?

The calculator uses the AASHTO formula: e + f = V² / (127 × R) for metric units, where e is superelevation rate, f is side friction factor, V is speed in km/h, and R is radius in meters. Rearranged: e = V² / (127 × R) − f.

What is a typical maximum superelevation rate?

AASHTO recommends e_max of 4–8% for highways. Rural roads in areas without snow or ice may use up to 10–12%. Urban streets typically cap at 4–6% to avoid hazards when vehicles are stopped.

What does a negative superelevation result mean?

A negative result means the curve is gentle enough that friction alone can handle the centrifugal force — no banking is required. The road can use a normal crowned cross-section.

What side friction factor should I use?

Side friction factors depend on speed. AASHTO values range from about 0.14 at 120 km/h to 0.35 at 30 km/h. Lower speeds allow higher friction; higher speeds require lower assumed friction for safety.

Superelevation Calculator

Name: Superelevation Calculator
Availability: InStock
Author: Nham Vu

Calculate road superelevation (cross-slope) for a horizontal curve given design speed, radius, and friction factor.

Curve Parameters

Design Speed (km/h)

Curve Radius (m)

Side Friction Factor (f) (0.08 – 0.40)

Maximum Superelevation e_max (%) (4 – 12)

Results

Enter parameters and click Calculate.

Cross-Section Diagram

Side view of road cross-section showing banking

AASHTO Friction Factor Reference

Speed (km/h)	Speed (mph)	f (max)

Summary

Calculate road superelevation (cross-slope) for a horizontal curve given design speed, radius, and friction factor.

How it works

Enter the design speed of the road in km/h or mph.
Enter the horizontal curve radius in meters or feet.
Optionally adjust the maximum superelevation rate (e_max) and side friction factor.
The calculator applies the AASHTO formula: e = v² / (127 × R) − f to find required superelevation.
Results show required superelevation rate, banking angle in degrees, and a safety assessment.

Use cases

Highway and road design projects requiring curve geometry checks.
Verifying existing road superelevation against posted speed limits.
Teaching transportation engineering concepts in academic settings.
Preliminary design checks for roundabouts and interchange ramps.
Checking compliance with AASHTO or local road design standards.
Mountain road and rural highway curve safety assessments.

Frequently Asked Questions

Last updated: 2026-06-11 · Reviewed by Nham Vu