Where cuI-de-sacs are provided at the end of a street with a relatively flat grade, the same design considerations are given as with other types of streets. Dry-weather flow into the cul-de-sac from adjacent property must be confined to the gutter, and provisions must be made for its disposal.
If the cuI-de-sac area is located at the high end or at the low end of a steep street, the grade of the cul-de-sac area should be reduced. This flatter area reduces the overturning effect on a vehicle making its turning movement within the cul-de-sac area. The flatter area also helps to control drainage by reducing the velocity of water flow. This reduced velocity will minimize the chances of water jumping over a curb or leaving the confines of the gutter when the water reaches a change in cul-de-sac alignment.
Vertical curves should be used to provide a smooth transition between the steep street grade and the reduced cul-de-sac grade. The length of vertical curve should be based on a vehicle speed of 25 miles per hour. For cul-de-sacs located at the high end or low end of a steep street, the charts that should be used for determining the vertical curve length are in sub-sections E 442.1, Stopping Sight Distance on Crest Vertical (Summit) Curves, and E 442.2, Headlight Sight Distance on Sag Vertical Curves. It is presumed that vehicles will travel at a very low speed in turning around a cul-de-sac. Therefore, if necessary to meet existing improvements, a maximum curb break of 2 percent may be used at or near the high or low point (usually the point of grade reversal) of the curb grade around the cul-de-sac. The crown used for the cul-de-sac area is determined by the formulas shown in Subsection E 621.1C, Parabolic Crown Section. The result to the nearest 0.5-inch is 0.50 feet for the standard turning radius of 35 feet.
The maximum desirable crossfall across the cul-de-sac at a section that is perpendicular to the centerline of the street and produced across the cul-de-sac radius point is 2.5 percent from flow line to flow line.
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