WORLD FOR CIVIL

Transition (spiral) Curves

While taking a turn,the centifugal forces develop so a vehicle and its contents are immediately subjected to centrifugal forces. More is the speed of vehicle sharper is the curvature and thus the greater the influence on vehicles and drivers of the change from tangent to curve. When transition curves are not provided, drivers tend to create their own transition curves by moving laterally within their travel lane and sometimes the adjoining lane,which is risky not only for them but also for other road users.

Mathematically
The minimum length L, ft (m), of a spiral may be computed from

L=3.15V³/RC

where
V =vehicle velocity, mi/h (km/h)

R= radius, ft (m), of the circular curve to which the spiral is joined

C= rate of increase of radial acceleration

An empirical value indicative of the comfort and safety involved, C values often used for highways range from 1 to 3.