DESIGN OF HILL CURVES AND SAFETY MEASURES FOR PROVIDING PROTECTIVE RELIEF, REQUIRED TO AVOID ACCIDENTAL RISKS.

Road curves are mainly of two types--Horizontal curves and the vertical curves. A horizontal curve is a curve in plan provided to change the direction of an alignment, where as a vertical curve in elevation is provided at change of gradient. There are two classes of vertical curves (a) convex or summit curves and (b) concave or valley curves, also known as sag curves. The design of a summit curve is governed mainly by the sight distance, required to provide adequate visibility, unless the summit is so small, less than 0.90 mt, above the lowest point of the curve, as not to interfere with visibility. In practice a simple parabola curve is used instead of a circular arc for a vertical curve, which joins the two gradients gradually.when two slopes meet, the change from one to the other should be gradual. This is achieved by using a vertical curve to unite the two gradients. Simple curve is a single circular arc connecting two tangents whereas a compound curve is formed by two adjoining simple curves. Hair pin bends are circular curves with transition curves at each end. Compound circular curves can be provided instead of the hair pin bends, and it is necessary to follow design criterion required to adopted normally for the design of hair pin bends for which straight length between two successive hair pin bends should not be less than 60 mt minimum, excluding the length of circular and transition curves. In case of the village roads length of transition should not be less than 15 mt whereas superelevation in circular portion of the curve to be taken 1 in 10 however this can be reduced to 1 in 15 for light traffic roads.