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Vertical Alignment Design Suggestions
The following excerpt was taken from the 1994 edition of AASHTO's A
Policy on Geometric Design of Highways and Streets (pp. 293-294).
In addition to the . . . specific controls for vertical alignment, there are
several general controls that should be considered in design.
- A smooth gradeline with gradual changes, as consistent with the type of highways, roads,
or streets and the character of terrain, should be strived for in preference to a line
with numerous breaks and short lengths of grades. . . .
- The "roller-coaster" or the "hidden-dip" type of profile should be
avoided. Such profiles generally occur on relatively straight horizontal alignment where
the roadway profile closely follows a rolling natural ground line. . . .
- Undulating gradelines, involving substantial lengths of momentum grades, should be
appraised for their effect on traffic operation. Such profiles permit heavy trucks to
operate at higher overall speeds than is possible when an upgrade is not preceded by a
downgrade, but may encourage excessive speeds of trucks with attendant conflicts with
other traffic.
- A broken-back gradeline (two vertical curves in the same direction separated by short
section of tangent grade) generally should be avoided, particularly in sags where the full
view of both vertical curves is not pleasing. . . .
- On long grades it may be preferable to place the steepest grades at the bottom and
lighten the grades near the top of the ascent or to break the sustained grade by short
intervals of lighter grade instead of a uniform sustained grade that might be only
slightly below the allowable maximum. . . .
- Where at-grade intersections occur on roadway sections with moderate to steep grades, it
is desirable to reduce the gradient through the intersection. . . .
- Sag vertical curves should be avoided in cuts unless adequate drainage can be provided.
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