The following are the important types of curves provided on hill Roads:-
- Hair-Pin Curves ;
- Salient Curves ;
- Re-entrant Curves.
1. Hair-pin curves:
The curve in a hill road which changes its direction through an angle of 180 degree or so, down the hill on the same side is known as hair-pin curve.
This curve is so called because it conforms to the shape of a hair-pin. The bend so formed at the hair-pin curve in a hill road is known as hair-pin bend. This type of curve should be located on a hill side having the minimum slope and maximum stability. It must also be safe from view point of land slides and ground water. Hair-pin bends with long arms and farther spacing are always preferred. They reduce construction problems and expensive protective works. Hair-pin curves or bends of serpentine nature are difficult to negotiate and should, therefore, be avoided as far as possible.
2. Salient curves:
The curves having their convexity on the outer edges of a hill road are called salient curves.
The centre of curvature of a salient curve lies towards the hill side. This type of curve occurs in the road length constructed on the ridge of a hill. the bend so formed at the salient curve in a hill road known as corner bend.
Salient curves are very dangerous for fast moving traffic. At such a curve or at corner bend, the portion of projecting hill side is usually cut down to improve the visibility as shown in fig. (re-entrant curve). The outer edge of the road at such a curve is essentially provided with a parapet wall for protection of the vehicles from falling downthe hill slope.
3. Re-entrant curves
The curves having their convexity on the inner edge of a hill road are called re-entrant curves.
The centre of curvature of a re-entrant curves lies away from the hill side. This type of curve occurs in the road length constructed in the calley of a hill. These curves are less dangerous as they provide adequate visibility to the fast moving traffic. At such curves, the parapet wall is provided only for safety of fast moving traffic.