Safe Curve Speed from Radius and Superelevation
The inverse of the superelevation tile: 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.
Formula and source
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.
Audience
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Posture
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