Calculator Inputs
This tool estimates required superelevation, adopted superelevation, edge rise, runoff length, tangent runout, and total transition length.
Formula Used
e = (V² / 127R) − f
e = (V² / 15R) − f
Edge Rise = e × W
Runoff Length = W × e × N
Tangent Runout = W × n × N ÷ 2
Here, e is superelevation as a decimal, V is design speed, R is curve radius, f is side friction factor, W is rotated pavement width, N is the relative gradient denominator, and n is normal crown rate as a decimal.
The calculator first finds the required superelevation. It then caps the adopted value at the chosen maximum rate, which is common in roadway design practice.
How to Use This Calculator
- Select the unit system that matches your project data.
- Enter design speed and the horizontal curve radius.
- Enter the side friction factor used by your design standard.
- Set the maximum allowed superelevation rate.
- Enter the rotated pavement width across the traveled way.
- Provide the normal crown rate and relative gradient denominator.
- Click the calculate button to view the result section above the form.
- Use the export buttons for a CSV summary or PDF report.
Example Data Table
| Case | Units | Speed | Radius | Friction | Max e | Width | Normal Crown | 1:N | Adopted e |
|---|---|---|---|---|---|---|---|---|---|
| Urban Collector | Metric | 60 km/h | 250 m | 0.13 | 8% | 7.0 m | 2% | 150 | 0.343% |
| Highway Ramp | Metric | 80 km/h | 180 m | 0.12 | 8% | 8.4 m | 2% | 150 | 8.000% |
| Rural Curve | Imperial | 45 mph | 900 ft | 0.11 | 6% | 24 ft | 2% | 180 | 4.000% |
Frequently Asked Questions
1. What does superelevation mean in road construction?
Superelevation is the cross slope applied on a horizontal curve. It raises the outer edge of pavement to help counter lateral vehicle forces and improve comfort, drainage, and safety.
2. Why does the calculator cap the adopted rate?
Road agencies often set a maximum superelevation value. This limit reflects climate, speed environment, maintenance concerns, and vehicle behavior. The calculator shows when the required value exceeds that chosen limit.
3. What is the side friction factor?
Side friction factor represents the share of lateral force resisted by tire-pavement friction. It works together with superelevation and affects how much cross slope is required for a given speed and radius.
4. What is runoff length?
Runoff length is the distance needed to rotate the pavement from a flat or crown-removed section into full superelevation. It helps achieve a smoother and more constructible transition.
5. Why is pavement width included?
The width controls the edge rise and transition lengths. Wider rotated sections need more vertical change and typically require longer runoff to maintain acceptable relative gradients.
6. Should I use metric or imperial values?
Use the unit system required by your project standard. The calculator switches constants and labels automatically, so the entered speed, radius, and width remain consistent with the selected system.
7. Does this replace roadway design standards?
No. It is a planning and checking tool. Final design should always follow the controlling local standard, including limits for friction, runoff, drainage, multilane rotation, and constructability.
8. What does the Plotly graph show?
The graph compares required and adopted superelevation rates across a range of curve radii at your selected speed and friction. It helps visualize how sharper curves demand larger cross slopes.