Road Superelevation and Runoff Calculator
Enter parameters and click "Calculate" to see results
What is Road Superelevation and Runoff Calculator?
Superelevation (e) is the transverse slope provided on a horizontal curve to counteract the centrifugal force acting on vehicles, allowing safe negotiation of the curve at design speed without relying entirely on tire side friction. The Superelevation & Runoff Calculator computes the required superelevation rate (e_req), validates a provided superelevation (e_prov), calculates tangent runout (Lt) and superelevation runoff length (Lr), and checks overall safety and compliance for any road curve.
This superelevation calculator online, AASHTO superelevation calculator, IRC superelevation runoff length calculator, highway curve banking calculator, tangent runout and runoff calculator, safe superelevation design tool, and road curve geometry calculator is built for precise, deterministic calculations under AASHTO Green Book, IRC:73, Austroads, or custom standards. It provides relevant visualizations (cross-section diagram with superelevation slope, runoff transition diagram, safety gauge), a dedicated section for comments, analysis and recommendations, full step-by-step calculation with every equation shown, CSV export/download of results (station-wise elevations, runoff table), and a Colorblind view for increased accessibility.
How to use Superelevation & Runoff Calculator
Purpose: Determine the exact superelevation needed for safe curve design OR validate an existing superelevation value, then compute the required transition lengths (tangent runout + runoff) so the curve meets comfort, safety, and drainage criteria.
Inputs you will enter:
- Design mode: Calculate Superelevation / Validate Superelevation / Geometry Only
- Design standard: AASHTO / IRC / Austroads / Custom
- Design speed V (km/h or mph)
- Curve radius R (m or ft)
- Rotated width B (m or ft) – pavement width being rotated
- Provided superelevation e_prov (%) – only for Validate mode
- Normal crown cross-slope (optional)
- Runoff distribution (Tangent only / Split / Curve only)
- Superelevation runoff rate a (m/m or ft/ft) – default from standard
Superelevation Formula
Required superelevation (SI units) \(e_{req} = \frac{V^2}{127R} – f_{used}\)
Tangent runout length \(L_t = \frac{n \times B}{a}\)
Superelevation runoff length \(L_r = \frac{e_{used} \times B}{a}\)
Where:
- V = design speed (km/h)
- R = curve radius (m)
- f_used = side friction factor used (≤ f_max from standard)
- B = rotated pavement width (m)
- a = superelevation runoff rate (m/m)
- n = number of lanes being rotated
- e_used = e_req (Calculate mode) or e_prov (Validate mode)
How to Calculate Superelevation & Runoff (Step-by-Step)
- Select Design Mode (Calculate / Validate / Geometry Only).
- Choose design standard (AASHTO / IRC / Austroads / Custom).
- Enter design speed V, radius R, and rotated width B.
- If Validate mode → enter provided superelevation e_prov (%).
- Calculator computes e_req using the selected standard’s f_max.
- Compares e_req vs e_max and e_prov vs safety criteria.
- Computes tangent runout Lt and runoff Lr based on user-selected distribution.
- Generates station-wise transition table, cross-section diagram, and PASS/FAIL verdict with recommendations.
Examples
Example 1 – Calculate Mode (AASHTO Rural Highway) V = 100 km/h, R = 400 m, B = 7.5 m (two lanes), standard = AASHTO e_req = (100² / (127×400)) – 0.12 = 0.0787 – 0.12 = –0.0413 → capped at 0 (no superelevation needed) Lr = 0 (flat curve) → PASS
Example 2 – Validate Mode (IRC Plain Terrain) V = 80 km/h, R = 250 m, e_prov = 7 %, B = 7.0 m e_req = (80² / (127×250)) – 0.15 ≈ 0.202 – 0.15 = 0.052 (5.2 %) Provided 7 % > 5.2 % and ≤ e_max = 7 % → SAFE Lr = (0.07 × 7.0) / 0.01 = 49 m (assuming a = 1:100) → PASS
Superelevation Categories / Normal Range
| Design Speed (km/h) | AASHTO e_max | IRC e_max (Plain) | Typical f_max | Minimum Radius for e=0 (m) |
|---|---|---|---|---|
| 50 | 0.06–0.08 | 0.07 | 0.15–0.17 | 140 |
| 80 | 0.08 | 0.07 | 0.14 | 360 |
| 100 | 0.08–0.10 | 0.07 | 0.12 | 650 |
| 120 | 0.10–0.12 | 0.07 | 0.09 | 1,050 |
Limitations
- Strictly follows selected standard; no automatic adjustment of radius or speed.
- Does not perform comfort (lateral acceleration) or drainage checks unless explicitly enabled.
- Runoff rate a is user-selected or standard default; no automatic calculation from superelevation rate.
- Geometry Only mode skips all safety calculations.
- Mixed units are allowed but must be declared; internal engine enforces SI.
Disclaimer
This calculator is provided for educational purposes, learning, and preliminary highway geometric design checks only. All final superelevation and vertical/horizontal alignment designs must be reviewed and certified by a qualified professional highway/traffic engineer as per the governing design code and project specifications. The developer and platform are not liable for any errors, misinterpretations, or consequences arising from the use of these results in actual road construction projects.