The following excerpt about the combination of horizontal and
vertical alignments in roadway design was taken from page 297 of the 1990 edition of
AASHTO's A Policy on Geometric Design of Highways and Streets.
Coordination of horizontal alignment and profile should not be left to chance but
should begin with preliminary design, during which stage adjustments can readily be made.
Although a specific order of study for all highways cannot be stated, a general procedure
applicable to most facilities can be outlined.
The designer should use working drawings of a size, scale and arrangement so that he
can study long, continuous stretches of highway in both plan and profile and visualize the
whole in three dimensions. Working drawings should be of sufficiently small scale,
generally 1 inch = 100 feet or 1 inch = 200 feet with the profile plotted jointly with the
plan. A continuous roll of plan-profile paper usually is suitable for this purpose.
After study of the horizontal alignment and profile in preliminary form, adjustments in
each, or both, can be made jointly to obtain the desired coordination. At this stage the
designer should not be concerned with line calculations other than known major controls.
The study should be made largely on the basis of a graphical analysis...
The coordination of horizontal alignment and profile from the viewpoint of appearance
usually can be accomplished visually on the preliminary working drawings. Generally, this
visual method results in a satisfactory product when done by an experienced designer. This
means of analysis may be supplemented by models or perspective sketches at locations where
the effect of certain combinations of line and grade are questionable.