The average speed accepted for normal usage is 67 of the road design speed in

# The average speed accepted for normal usage is 67 of

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The ‘average speed’ accepted for normal usage is 67% of the road design speed , in practice. Generally, superelevation should never exceed 7%, with 5% being the maximum in urban areas.
Horizontal Alignment Lecture 4 13/04/14 15 Methods of Building Superelevation On straight sections of roads a camber on both sides from the crown or centerline is normally provided. On horizontal curves, however, the road has to be superelevated (I.e. outer edge has to be raised relative to the inner edge) to ensure vehicle stability and safety. Thus a cambered section is changed into a superelevated one. This can be accomplished in two stages: Firstly, the camber on the outer edge is neutralised gradually till the road section has one straight line slope from the inner to the outer edge. Secondly, this straight line slope is gradually increased till the designed superelevation is achieved There are two (2) principal techniques for effecting these changes:
Horizontal Alignment Lecture 4 13/04/14 16 By rotating the outer edge slowly round the crown, I.e. by progressively decreasing the inclination of the outer edge slope tangential to the curved crown till it reaches the horizontal and then slowly increasing it until it has the same inclination as the inner slope. Disadvantage The main disadvantage of this method is the difficulty of draining water from the surface of the road for some length. As the outer slope has first to be changed from the cambered section to straight line, there will be some length of the road which will have a slope on the outer edge less than the camber which is absolutely required for draining the road surface effectively. This problem may be mitigated if the road has a good longitudinal fall. Advantages in using this method are : Where no transitions have been provided at the ends of the curves, the outer half of the road can be brought to level before the start of the curve (tangent point). Thus at no point in the curve surface will there be a negative superelevation, I.e. it will not slope downwards. In the case of transition curves, although there will be a negative slope for some portion, but this decreases as the radius of curvature diminishes. It does not require the use of special templates along the transition length
Horizontal Alignment Lecture 4 13/04/14 17 The second technique is the ‘diagonal crown method’, I.e. by shifting of the crown position to the outer edge. Thus the cambered profile is eliminated by progressively shifting the crown towards the outer edge and along the extension of the inner slope. This is continued till the crown reaches the outer edge. The camber on both sides of the crown will remain unchanged throughout the transformation . Advantage The method is ideal for use in dead flat terrain where drainage cannot be effected by the longitudinal fall of the road as the camber required for the drainage is not interfered with.

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