Sag Vertical Curve Comfort and Drainage (AASHTO)
The comfort and drainage sag-curve controls the headlight tile names as separate checks. The AASHTO comfort criterion L = A V^2/46.5 (A in %, V in mph) is the length that holds the vertical acceleration on the sag to about 1 ft/s^2, giving K = L/A = V^2/46.5; the drainage maximum K <= 167 (a 0.30% minimum grade within 50 ft of the low point, 50/0.30) caps the length at 167 A on curbed sections. A 4% break at 60 mph wants at least a 310 ft curve (K 77.4) for comfort and no more than 668 ft for drainage. Comfort is far less restrictive than headlight SSD; a licensed civil engineer's design governs.
Formula and source
Comfort: L = A V^2 / 46.5 (A in %, V in mph, L in ft), K = L/A = V^2/46.5. Drainage maximum K <= 167 (a 0.30% minimum grade within 50 ft of the low point, 50/0.30 = 166.7), so the drainage-max length is 167 A.
The AASHTO A Policy on Geometric Design of Highways and Streets (the Green Book) sag vertical-curve comfort and drainage controls, by name.
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