Practice Question: Length of Crest Curve Based on Stopping Distance

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Solution

Answer: B) 455 ft

The required curved length for stopping sight along a crest curve can be calculated based on requirements found in the AASHTO document A Policy on Geometric Design of Highways and Streets.
Define design parameters:
$$ A=absolute\ value\ of\ grade\ difference $$
$$ S=stopping\ sight\ distance $$
$$ L=length\ of\ curve $$

If the stopping distance is less than the curve length (S < L), curve length can be calculated as:
$$ L=\frac{A\ast S^2}{2158} $$
If the stopping distance is greater than the curve length (S > L), curve length can be calculated as:
$$ L=2\ast\ S-\frac{2158}{A} $$
where:
$$ A=\left|G_2-G_1\right| $$

$$ G_1 = 3.0% $$

$$ G_2 = -5.0% $$

$$ A=8.0% $$
Assume S < L:
$$ L=\frac{8.0\ast\left(350\ ft\right)^2}{2158}\approx455\ ft>S=350\ ft\rightarrow\ confirmed $$
$$ L=455\ ft $$

 

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