1. To avoid certain religious places, monuments of Historic importance etc..
2. To provide access to a certain locality (villages or town).
3. To preserve the certain existing amenities.
4. To solve the problems of land acquisition. Types of Horizontal Curves
1. Circular curves
(a) Simple curves
(b) Compound curves
(c) Reverse curves
2. Transition curves
Circular Curves
1. Simple Curves: A simple curve is of single arc of uniform or same radius connecting two tangents Simple curves are suitable for slow moving traffic and large radius.
(b) Compound Curves: A compound curve is formed by joining two or more simple curves of different radii but their centres should lie on same side of the curve Simple curves have a common tangent Compound curves may be used in difficult topography (to avoid cutting hard rock) but only when it is not possible to fit in a single circular curve.
(c) Reverse Curve (or Serpentine Curve): Reverse curve consists of two simple curves of same or different radii but their centre lie on opposite sides of the curve. In this curve, the simple curves have a common tangent.Reverse curves are provided in difficult terrain. It should be ensured that there is sufficient length between the curves for introduction of requisite transition curves.
Transition Curves:
These are necessary for a vehicle to have smooth entry from a straight section into a circular curve. The transition curves also Improve aesthetic appearance of the road besides permitting gradual application of the superelevation and extra widening of carriage way needed at horizontal curves.
Types of Transition Curves:
1. Spiral
2. Cubic parabola
3. Leminiscate
Objects of Providing a Transition Curves:
1. To provide comfort to road users
2. To provide gradual change in the radius of curvature
VERTICAL CURVES
Vertical curves should provide for a smooth longitudinal profile consisting with category of the road and lay of the terrain. Grade changes should not be too frequent as it cause kinks and visual distortions in the profile.
Objects of Vertical Curves:
1. To provide gradual change in elevation
2. To provide sufficient visibility
3. To provide comfort for road users.
Types of Vertical Curves
1. Summit curves
2. Valley curves
1. Summit Curves: These curves have their convexity upwards.
(a) When a rising gradient intersects a falling gradient
(b) When a rising gradient meets another rising gradient
(c) When a rising gradient meets a horizontal or level road.
(d) When a falling gradient meets a steeper falling gradient.
2. Valley Curves: These curves have their concavity upwards
(a) When a falling gradient intersects a rising gradient
(b) When a falling gradient meets another falling gradient
(c) When a falling gradient meets with a horizontal or level road.
(d) When a rising gradient meets steeper rising gradient.
