The following excerpt was taken from page 328 of the 1990 edition of AASHTO's A Policy on Geometric Design of Highways and Streets.
Two-lane and wider undivided pavements on tangents or on flat curves have a crown or high point in the middle and slope downward toward both edges. The downward cross slope may be a plane or curved section or a combination of the two. With plane cross slopes, there is a cross slope break at the crown line and a uniform slope on each side. Curved cross sections usually are parabolic, with a slightly rounded surface at the crown line and increasing cross slope toward the pavement edge. Because the rate of crown slope is variable, the parabolic section is described by the crown height, i.e., the vertical drop from the center crown line to the pavement edge. The advantage of the curved section lies in the fact that the cross slope steepens toward the pavement edge, thereby facilitating drainage. The disadvantages are that curved sections are more difficult to construct, the cross slope of the outer lanes may be excessive, and warping of pavement areas at intersections may be awkward or difficult to construct.
On divided highways, each one-way pavement may be crowned separately, as on two-lane highways, or it may have a unidirectional slope across the entire width of pavement, which is almost always downward to the outer edge. Where freeze-thaw conditions are a problem, each pavement of a divided highway should be crowned separately.