80 km/h, R = 1500 m, emax = 6%
Table 3-9, 80 km/h column: R 1530 m → e 2.2%, R 1380 m → e 2.4%. Interpolating, e = 2.24% → design e = 2.2%.
AASHTO superelevation tables
Superelevation Rate Calculator – AASHTO Tables
Find the AASHTO design superelevation rate for a horizontal curve from design speed, radius and emax 4%, 6% or 8% (Green Book 2018 tables).
Pick emax (4%, 6% or 8%), the design speed and the curve radius. The tool reads the design superelevation rate from the metric AASHTO Green Book (7th Edition, 2018) tables — Method 5 distribution — interpolating between the table radii, and tells you when the radius is below the minimum or when normal crown (NC) or reverse crown (RC) applies.
Maximum superelevation, emax
Design speed and radius
Superelevation e
—
%
AASHTO table
Columns are design speeds Vd (km/h); values are minimum radii R (m) for each design superelevation rate e (%). NC = normal crown (no superelevation needed); RC = remove adverse crown — superelevate the whole section at the normal cross slope. Highlighted: your design speed column and the table radii used for your result.
Method & source
For the chosen emax and design speed the tool takes the AASHTO table column of minimum radii and finds the two rows whose radii bracket your radius R. The superelevation rate is interpolated linearly between those two rows and rounded to 0.1%.
- R ≥ the NC radius → normal crown, no superelevation.
- R between the RC and NC radii → reverse crown: superelevate at the normal cross slope (the tables are based on a 2% normal cross slope, so RC is taken as e = 2.0% when interpolating up to the 2.2% row).
- R below the emax row (Rmin) → not allowed for that design speed and emax; use a larger radius or a lower design speed.
Source: AASHTO, A Policy on Geometric Design of Highways and Streets (Green Book), 7th Edition, 2018 — metric Tables 3-8 (emax = 4%, 20–100 km/h), 3-9 (emax = 6%) and 3-10 (emax = 8%), Minimum Radii for Design Superelevation Rates, Design Speeds, and emax (Method 5 distribution of e and f). The table values in this tool were read directly from the printed tables, not recalculated.
The Green Book notes that emax = 4% should be limited to urban areas. This tool is for design and checking; confirm against the governing agency's design manual before finalizing a design.
Method & practical use
Understand the result.
Find the AASHTO design superelevation rate for a horizontal curve from design speed, radius and emax 4%, 6% or 8% (Green Book 2018 tables).
How to use this tool
- Choose emax — 4% (urban), 6% or 8% — as set by your design standard.
- Pick the design speed and type the curve radius in metres; the result updates as you type.
- Press View Table to see the AASHTO table with your values highlighted, then print or save the result as PDF.
Units & method
Speed km/h; radius m; superelevation e in % of the cross slope.
Linear interpolation between the AASHTO table radii for the chosen design speed and emax (Method 5), rounded to 0.1%. NC, RC and Rmin limits come straight from the same table column.
Worked examples
Illustrative inputs for checking the method; these are not project records.
60 km/h, R = 300 m, emax = 8%
Table 3-10, 60 km/h column: R 307 m → e 5.4%, R 288 m → e 5.6%. Interpolating, e = 5.47% → design e = 5.5%.
50 km/h, R = 400 m, emax = 4%
Table 3-8, 50 km/h column: R 435 m → e 2.4%, R 345 m → e 2.6%. Interpolating, e = 2.48% → design e = 2.5%.
Frequently asked questions
What does this tool calculate?
Find the AASHTO design superelevation rate for a horizontal curve from design speed, radius and emax 4%, 6% or 8% (Green Book 2018 tables). It also flags radii below the minimum and radii large enough for normal crown or reverse crown.
Why is the result interpolated?
The AASHTO tables give the minimum radius for each 0.2% step of superelevation. A radius between two table radii gets a rate between the two table rates; the tool interpolates linearly and rounds to 0.1%. You can always round up to the next table rate if your agency prefers.
Can I print or save the result?
Yes. Print / Save as PDF prints the inputs, the result and the AASHTO table column used. Choose Save as PDF as the printer to get a PDF.