Superelevation Calculator
Calculate highway curve superelevation rate, bank angle, minimum curve radius, and maximum safe speed using AASHTO design formulas.
Inputs
AASHTO Table 3-7: 20 mph f=0.18; 40 mph f=0.15; 60 mph f=0.12; 80 mph f=0.08
Results
Superelevation Rate (e)
0 ft/ft
Min Radius (e_max=8%)
614 ft
≈ 8 tennis courts
How to Use This Calculator
- Enter design speed and horizontal curve radius.
- Set maximum superelevation rate per your jurisdiction (AASHTO typically 4–8%).
- Review required superelevation (e) and transition length.
How the result changes with Design Speed
| Design Speed | Superelevation Rate (e) | Min Radius (e_max=8%) |
|---|---|---|
| 22 | 0 ft/ft | 147 ft |
| 38 | 0 ft/ft | 438 ft |
| 57 | 0.08 ft/ft | 985 ft |
| 74 | 0.12 ft/ft | 1,659 ft |
What each input means
- Design Speed
- Design speed of the roadway. Determines superelevation rate needed for safe operation through curves.
- Curve Radius
- Radius of the horizontal curve. Measured to the centerline of the roadway.
- Side Friction Factor (f)
- Maximum side friction demand per AASHTO Green Book Table 3-7. Friction decreases at higher speeds: 20 mph→0.18; 40 mph→0.15; 60 mph→0.12; 70 mph→0.10; 80 mph→0.08.
How this is calculated
Worked example, using the default values
- Identify Input ParametersDesign Speed = 45, Curve Radius = 1000, Side Friction Factor (f) = 0.14 = 3 input(s) provided
- Calculate Superelevation RateSuperelevation Rate0 = 0
- Calculate Min RadiusMin Radius614 = 614
- Calculate SuperelevationSuperelevation = superelevationRate0 = 0
- Calculate Bank AngleBank Angle0 = 0
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