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 Geometric Design: Professional Practice

 
Crest Vertical Curves

The following excerpt was taken from the 1994 edition of AASHTO's A Policy on Geometric Design of Highways and Streets (p. 282).

Minimum lengths of crest vertical curves as determined by sight distance requirements generally are satisfactory from the standpoint of safety, comfort, and appearance. An exception may be at decision areas, such as sight distance to ramp exit gores, where longer lengths are necessary. Refer to the section in this chapter concerning decision sight distance.

The basic formulas for length of a parabolic vertical curve in terms of algebraic difference in grade and sight distance follow:

When S is less than L,

L=(A*S^2)/(100*((2*h1)^0.5 + (2*h2)^0.5)^2)

When S is greater than L,

L=2*S-(200*(h1^0.5 + h2^0.5)^0.5)/A

Where:
L = length of vertical curve, m;
S = sight distance, m;
A = algebraic difference in grades, percent;
h1 = height of eye above roadway surface, m (normally 1070 mm/1000 mm/m)
h2 = height of object above roadway surface, m (normally 150 mm/1000 mm/m)

(The heights h1 and h2 given above correspond to the stopping sight distance.)