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Section 5: Descending Grades of Six Percent or Greater

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Where vehicles may attain a speed in excess of that required for a safe stopping distance or in excess of that required for safe travel around curves at the bottom or within the limits of the grade, the posting of an advisory speed may be prudent.

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Determining Minimum Sight Distances

The minimum horizontal and vertical sight distances on descending grades of 6 percent or greater should be determined either by field measurements or by calculations from the plans.

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Using the minimum sight distance as the safe stopping distance, the critical speed should be calculated from the formula

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d = sight distance for safe stopping distance in feet

V = the velocity in miles per hour

g = percent of grade (in decimal form) divided by 100.

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If a Curve is Involved

If a curve is within the limits of or at the bottom of such a grade, the advisory speed for the curve should be determined by the procedure found in Section 2 of this chapter, Curves and Turns

The speed to be posted should be the lower of the two speeds found for providing a safe stopping distance and that for safe travel around the curve.

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