A four-lane divided rural highway has a 70 mph design speed. The minimum length of a crest vertical curve connecting a +2.0% and -1.5% grade for stopping sight distance (SSD) is most nearly:

Why is this the answer?

For 70 mph, AASHTO Green Book SSD is 730 ft. A = 2.0 - (-1.5) = 3.5%. For crest vertical curves, if S < L, L = AS^2 / 2158 = 3.5 * 730^2 / 2158 = 864 ft. If S > L, L = 2S - 2158/A = 2(730) - 2158/3.5 = 1460 - 616.6 = 843 ft. Since 843 > 730, the S > L case applies. The minimum length is approximately 843 ft. Given the options provided, the original question parameters or options are inconsistent with AASHTO standards.

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