Safe Curve Speed from Radius and Superelevation

The maximum safe speed a curve supports from its radius, superelevation (bank), and side-friction factor, V = sqrt( 15 R (e + f) ) mph. A 1,500 ft radius at e 0.08 and f 0.12 supports about 67 mph. Use the AASHTO f for the resulting speed band (f decreases with speed; iterate once). Point-mass model; ignores grade and the transition. A design/check aid; the civil engineer governs.

V = sqrt( 15 R (e + f) ) mph, the inverse of the AASHTO point-mass e + f = V^2/(15 R). R in ft.

The AASHTO A Policy on Geometric Design of Highways and Streets (the Green Book) point-mass curve model, solved for the speed, by name.

The 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.

Estimate. AHJ and licensed professional govern.

Field names used by the API: R_ft, e, f, v_mph

• Point-mass model V = sqrt( 15 R (e + f) ), V in mph and R in ftAASHTO Green Book
• Side-friction factor use the AASHTO f for the resulting speed band (about 0.16 at 30 mph to 0.08 at 80 mph); iterate onceAASHTO Green Book speed table
• Not a runoff/spiral design ignores grade, the transition/spiral, and driver comfortscope of this tile