The following excerpt was taken from page 226 of the 1990 edition of
AASHTO's A Policy on Geometric Design of Highways and Streets.
The topography of the land traversed has an influence on the alignment of roads and
streets. Topography does affect horizontal alignment, but it is more evident in the effect
on vertical alignment. To characterize variations, engineers generally separate topography
into three classifications according to terrain. Level terrain is that condition where
highway sight distances, as governed by both horizontal and vertical restrictions, are
generally long or could be made to be so without construction difficulty or major expense.
Rolling terrain is that condition where the natural slopes consistently rise above and
fall below the road or street grade and where occasional steep slopes offer some
restriction to normal horizontal and vertical roadway alignment.
Mountainous terrain is that condition where longitudinal and transverse changes in the
elevation of the ground with respect to the road or street are abrupt and where benching
and side hill excavation are frequently required to obtain horizontal and vertical
alignment.
Terrain classification pertains to the general character of a specific route corridor.
Routes in valleys or passes or mountainous areas that have all the characteristics of
roads or streets traversing level or rolling terrain should be classified as level or
rolling. In general, rolling terrain generates steeper grades, causing trucks to reduce
speeds below those of passenger cars, and mountainous terrain aggravates the situation,
resulting in some trucks operating at crawl speeds.