How is the minimum length of a crest vertical curve determined?

Crest vertical curve length for stopping sight distance: L = A*S^2 / (100*(sqrt(2h1) + sqrt(2h2))^2) when S<L, where A = algebraic grade difference (%), S = sight distance, h1 = eye height (1.08m), h2 = object height (0.6m). For S>L: L = 2S - (100*(sqrt(2h1)+sqrt(2h2))^2)/A. Simplified: L = A*S^2/658 (S<L) or L = 2S - 658/A (S>L). Length ensures driver can see object at stopping sight distance over the curve crest.