A compound curve (Fig. 13.14) is formed by the combination of two circular curves of different radii curving in the same direction. A common transition curve may be provided between the two circular curves of a compound curve. Assuming that such a connecting curve is to be traversed at a uniform speed, the length of the transition curve connecting the two circular curves can be obtained from the formula
L = 0.008 (Cal - Ca2) * V
L = 0.008 (Cd1 - Cd2) x Vm, whichever is greater where Ca1 and Cd1 are the cant and cant deficiency for curve 1 and Ca2 and Cd2 are the cant and cant deficiency for curve 2 in millimetres. L is the length of the transition curve, in m,and Vm is the maximum permissible speed in km/h.