Max Design Speed from Sight Distance
The inverse of the stopping-sight-distance tile: the fastest safe design speed given the available sight distance, the positive root of v^2/(30(f+g)) + 1.47 x t x v - SSD = 0 (AASHTO Green Book). With 490 ft of sight distance on dry level pavement (f 0.35, t 2.5 s) the max is 55 mph; a -6% downhill drops it to 51 mph. Sets a curve/crest advisory speed or checks a design speed against the sight line, instead of the distance for one speed. A design aid; the AASHTO Green Book and the state DOT govern.
Formula and source
v is the positive root of v^2 / (30 (f + g)) + 1.47 t_pr v - SSD = 0, i.e. v = (-b + sqrt(b^2 + 4 a SSD)) / (2 a) with a = 1/(30(f+g)), b = 1.47 t_pr. The inverse of SSD = 1.47 t v + v^2/(30(f+g)).
AASHTO Green Book (Policy on Geometric Design of Highways and Streets, 7th ed.) Chapter 3.
Audience
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Posture
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