Sag Vertical Curves. The main factor affecting the design of a sag vertical curve is headlight
sight distance. When a vehicle traverses an unlighted sag vertical curve at night,
the portion of highway lighted ahead is dependent on the position of the headlights
and the direction of the light beam. For design purposes, the length of roadway lighted
ahead is assumed to be the available stopping sight distance for the curve. In calculating
the distances for a given set of grades and a length of curve, the height of the headlight
is assumed to be 2 ft (0.61 m) and the upward divergence of the light beam is
considered to be 1°. Table 2.19 lists the calculated design stopping sight distance values
and the corresponding K values for design speeds from 20 to 70 mi/h (32 to 113 km/h).
As was the case with crest curves, the values shown are based on the assumption that
the curve is longer than the sight distance. In those cases where the sight distance
exceeds the vertical curve length, a different equation is used to calculate the actual
stopping sight distance provided as indicated in the table