Superelevation / Min Curve Radius (AASHTO)
AASHTO point-mass e + f = V^2/(15 R): required superelevation for a radius, or the minimum radius at a maximum bank.
Example
You enter
- Solve for e
- Design speed V (mph) 60
- Curve radius R (ft) 1500
- Side-friction factor f 0.12
You get
- E req 0.04
Checked against published worked examples from: FHWA.
Details, formula, and sources
Formula or tablee + f = V^2/(15 R); required e = V^2/(15 R) - f; minimum radius R_min = V^2/(15(e_max + f)). V in mph, R in ft.
Source and editionThe AASHTO A Policy on Geometric Design of Highways and Streets (the Green Book) point-mass curve model, by name.
Free accessThe AASHTO point-mass relation is published in the Green Book; the side-friction design factors are in its speed table. AHJ, state DOT design manual, and the licensed civil engineer of record govern.
What governsEstimate. AHJ and licensed professional govern.
API field namesmode, V_mph, R_ft, f, e_req
- Point-mass model e + f = V^2/(15 R), V in mph and R in ftAASHTO Green Book
- Side-friction factor f is the AASHTO design value at the design speed (about 0.20 at 30 mph to 0.08 at 80 mph)AASHTO Green Book speed table
- Not a runoff/spiral design no e/f distribution method, no transition or spiral lengthscope of this tile