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Rh following table, corresponding to the design speed assumed for the section. Preferred limits are shown in the column headed “Desirable maximum.”

Curvature of lower degree than that shown in the above table may be necessary to meet sight-distance requirements where a cut slope or other obstacle obstructs the view across the inside of a curve.

Transition curvature.—All horizontal circular curves on rural sections of the system sharper than 2 degrees shall be approached by transition curves of a length consistent with the design speed and sufficient to permit the attainment of full superelevation within the length of the transition.

Superelevation a curves.—All curves sharper than 1 degree shall be superelevated. The maximum superelevation shall be 0.12 foot per foot. Where snow and ice may be expected to cause a frequent slippery condition of the road surface, the maximum superelevation shall be 0.08 foot per foot. On all curves the superelevation shall be such as to counterbalance completely the centrifugal force of a vehicle traveling at three-fourths of the design speed of the road, except that it shall not exceed the above stated appropriate maximum limit.

Superelevation shall be attained gradually, and in such manner that the difference in slope between longitudinal profiles separated by the width of one lane shall be not greater than 1 in 200.

Sight distance.—On all rural sections of the system the design shall be such as to afford from a height of 4.5 feet above any point on the road surface, a continuously unobstructed view for the following minimum nonpassing sight distances to the top of an object 4 inches high placed on the road surface.

Within these sight distances passenger vehicles as presently constructed can be stopped from the speeds shown, and trucks and combinations can be stopped from speeds ranging from 35 miles per hour where the sight distance is 400 feet to about 50 miles per hour where the sight distance is 800 feet.