When a vehicle travelling on a straight course(i.e of infinite radius) enters a curve of finite radius, a sudden lateral jerk is felt on the vehicle due to the centrifugal force.
In order to avoid this, a curve having its radius varying gradually from an infinite to finite value is provided in between the straight course and circular curve, this curve is called the transition curve.
Objects Of Providing Transition Curve
Transition curve is provided to achieve the following objects:
1. To introduce gradually the centrifugal force between the tangent point and the beginning of the circular curve and thus reducing the sudden jerk on the vehicle.
2. To introduce gradually the designed superelevation and extra widening necessary.
3. To enable the driver to turn the steering gradually for his own comfort and security.
4. To avoid discomfort to the passengers.
5. To minimize wear on the pavement.
6. To improve the aesthetic appearance of the road.
Types of Transition Curve
The types of transition curves commonly adopted in horizontal alignment are:
c) Cubic parabola
The I.R.C has recommended the use of spiral transition curve in the horizontal alignment.
Essential Requirement of a Transition Curve
Following are the essential requirement of a transition curve:
1. Its radius of curvature should vary gradually from infinite value at the straight to a finite value at the starting point of the circular curve.
2. It should meet the straight and the circular curve tangentially.
3. The length of the transition curve should be such that the designed superelevation is attained at or before the junction with the circular curve.
4. The rate of increase of curvature and that of superelevation should be the same.