University of Virginia Library

Discussion:

Horizontal and vertical alignment are directly related and are
therefore designed as a ribbon having three dimensions. Horizontal
curves are contained within a single vertical curve to insure uniformity
of centrifugal force and friction. Vertical curves are also
a function of sight distance, gradient and headlight distance. Hans
Lorenz, an engineer experienced in design of German autobahns,
suggests a rule that the vertex of the horizontal curve should coincide
with the vertex of the vertical curve to avoid a rollercoaster
effect within a horizontal curve. F. W. Cron, an engineer of the
Bureau, points out that "most of the awkwardness of the highway
arises from failure to visualize the road in three dimensons during
the planning stage." Mr. Cron spent much of his career working
with us in the National Park Service.

Short vertical curves are to be avoided in any case even though
the vertex of the vertical coincides with the vertex of the horizontal
curve. Vertical curves often make up 50% of the profile and may be
greater on gentle topography. In the design of park roads, we strive
to make the appearance of the road forward as aesthetically pleasing
as geometry will permit. This criteria is made more difficult by
the requirement that cuts and fills be kept to an absolute minimum.

Virginia Highway's tables for minimum sight distance on vertical
curves show curves as short as 50 feet and as long as 2000 feet.
New York State requires a minimum sight distance of 1000 feet on
its Thruway. (Tunnard, page 193) My own opinion is that vertical
curves should not be shorter than 250 feet which is about the minimum
radius of a horizontal curve at 30 MPH (232 feet). The alternative
to shorter vertical curves is an elevated structure on sag curves;
summit curves are less difficult, usually, because landforms tend
to be rounded on convex slopes and sharper in valleys. Park roads
are by nature of low speed, and therefore sight distances are much
shorter than on high speed roadways. It is, after all, a principle
of controlling the speed at which drivers will travel the roadway
that park roads are built.

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