Transportation Engineering

It is the radius of the circular arc that connects the two tangent sections of the roadway; a larger radius gives a flatter, gentler curve.

D = 5729.58 / R, where D is the central angle (degrees) subtended by a 100-ft arc and R is in feet.

Superelevation (e) is the banking/cross-slope of the roadway across the curve; it lets a component of gravity help counteract centrifugal force so vehicles can negotiate the curve safely.

e + f = V^2 / (gR), often written e + f = V^2 / (15R) with V in mph and R in feet (U.S. units).

It increases with the square of speed; doubling speed quadruples the required e + f for the same radius.