How to use the Superelevation and Curve Design Calculator
- 1. Enter speed and radiusDesign speed and curve radius. Code values are editable inputs.
- 2. Set the roadPavement width, lanes, wheelbase and rotation about centre or inner edge.
- 3. Get the designSuperelevation, friction check, widening, raise of outer edge and transition.
IRC method for superelevation
Step 1: e = V²/(225R) (75% of design speed, no friction). Step 2: if e ≤ emax, provide it; otherwise provide emax. Step 3: check friction at full speed, f = V²/(127R) − e ≤ 0.15. Step 4: if f is more than 0.15, restrict speed to Va = √(127R(e + f)) or increase the radius.
Commonly used IRC values are emax = 0.07 for plain and rolling terrain and 0.10 for hill roads not bound by snow, with f = 0.15. Extra widening is nl²/(2R) + V/(9.5√R). Transition length is the largest of 0.0215V³/(CR) with C = 80/(75 + V), the rate of introducing superelevation (1 in 150 in plains), and the empirical 2.7V²/R. Check the current IRC codes before design.
Worked example and exam tips
V = 65 km/h, R = 220 m, width 7.5 m rotated about the centre line, 1 in 150: e = 65²/(225 × 220) = 0.085 > 0.07, so e = 0.07. C = 80/140 = 0.57, Ls = 0.0215 × 65³/(0.57 × 220) = 47.1 m; by superelevation 0.07 × 3.75 × 150 = 39.4 m; empirical 2.7 × 65²/220 = 51.9 m. Adopt Ls ≈ 52 m, shift = 52²/(24 × 220) = 0.51 m.
- Hill roads use 1 in 60 and the empirical coefficient 1.0. Change both inputs.
- Superelevation is never less than the camber.
- Round the transition length up for design.
Frequently asked questions
Why is 225 used instead of 127 in step 1?
Step 1 designs for 75% of the design speed with no friction. (0.75V)²/(127R) ≈ V²/(225R). This keeps e practical for slow mixed traffic.
What if friction needed is more than 0.15?
The curve is not safe at that speed. Either restrict the speed to √(127R(e + f)) or use a larger radius.
What is extra widening?
Extra pavement width on curves: mechanical widening nl²/(2R) for off-tracking of rear wheels, and psychological widening V/(9.5√R) for driver comfort.
Is this calculator free?
Yes. You fill a short form once to see your result, and other Uncodemy tools then open without the form. Everything is calculated in your browser and nothing you type is uploaded.