60 km/h, 6% superelevation
With f = 0.17, R = 60²/[127(0.06 + 0.17)] = 123.25 m.
Compute Min. Radius
Minimum Radius of Horizontal Curve Calculator
Estimate the minimum horizontal curve radius from design speed, superelevation and the displayed side-friction factor.
Enter a design speed and pick a maximum superelevation rate — the tool computes the AASHTO minimum radius using the side-friction curve from the Green Book (7th Edition, 2018), interpolated for any speed between 20 and 130 km/h.
Maximum superelevation, emax
Design speed
Radius (m)
—
metres
Reference table
Minimum radius (m) at common design speeds, for the selected emax — the row matching your last computed speed is highlighted.
Formula & source
Rmin = V² / [127 × (emax + fmax)]
V = design speed (km/h), emax = maximum superelevation rate (decimal), fmax = maximum side-friction factor for that speed (open-roadway condition, AASHTO Method 5 distribution).
Source: AASHTO, A Policy on Geometric Design of Highways and Streets (Green Book), 7th Edition, 2018 — Tables 3-7/3-8/3-9/3-10, Minimum Radii for Design Superelevation Rates. fmax values used here (0.35 at 20 km/h down to 0.10 at 120–130 km/h) are interpolated from the published metric tables for emax = 4%, 6%, and 8%, cross-checked against each other for consistency.
This tool is for preliminary design and checking. Always confirm against the current edition of the Green Book (or the governing agency's design manual) before finalizing a design.
Method & practical use
Understand the result.
Estimate the minimum horizontal curve radius from design speed, superelevation and the displayed side-friction factor.
How to use this tool
- Select the geometry or design case and confirm the displayed units.
- Replace sample inputs with the project values, then calculate or review the live result.
- Check the method, assumptions and output; save or print the calculation with its input record.
Units & method
Speed km/h; radius m; superelevation entered as %.
R = V²/[127(e + f)], using decimal e. The embedded speed/friction schedule is attributed by the original tool to AASHTO 2018; the exact licensed schedule is not independently certified here. Use the displayed friction factor and check the governing agency criteria.
Worked examples
Illustrative inputs for checking the method; these are not project records.
80 km/h, 6% superelevation
With f = 0.14, R = 80²/[127(0.06 + 0.14)] = 251.97 m.
Frequently asked questions
What does this tool calculate or track?
Estimate the minimum horizontal curve radius from design speed, superelevation and the displayed side-friction factor.
Which units and assumptions should I use?
Speed km/h; radius m; superelevation entered as %. R = V²/[127(e + f)], using decimal e. The embedded speed/friction schedule is attributed by the original tool to AASHTO 2018; the exact licensed schedule is not independently certified here. Use the displayed friction factor and check the governing agency criteria.
Can I save or share the result?
Use the save, export and print controls shown in this workspace. Where CSV export is available, the exported data can be opened in a spreadsheet. Browser print can save a PDF.