Civil Engineering Reference
In-Depth Information
Chapter 6
Road Geometry
Most parcels of land have frontage on public or private roads or streets so it is ben-
eficial to spend a little time reviewing typical road geometries. Calculating road
geometry requires an understanding of geometry and trigonometry so a compre-
hensive treatment of the subject is beyond the scope of this portion of the topic.
Nevertheless, the explanations found in this chapter will provide the reader with
a good foundation for understanding the geometry of roads and road curves. For
those wishing a more comprehensive discussion, the subject is treated in greater
depth in the appendix.
When moving automobiles change direction they do not do so instantaneously.
Direction is changed gradually by going around a curve. On a highway, where
vehicles are traveling at high speed, the curves are gradual so they can be negoti-
ated without changing speed. In the city where travel is slow, (or should be) and
streets intersect at right angles, curves are relatively abrupt or even non-existent.
The feature that makes a curve gradual or abrupt is known as the
Curve Radius
.
The longer the curve radius, the more gradual the curve is and it can be negotiated
at higher speed. The shorter the curve radius the more abrupt the curve is and it
must be negotiated at lower speed. The names
Road
and
Street
mean the same
thing in our context so the names will be used here interchangeably.
Roads are often described using the term
Road Layout
. A road layout is a plan
that describes the features of a road, such as road dimensions, intersections, curve
geometry, grades and other attributes. Layouts are often held in public recording
offices such as highway engineering departments and recording offices which hold
plans and deeds. In some cases, when roads are newly created, widened or relo-
cated, engineers working for the municipality or state will design the road, create
plans, lay it out on the ground and set monuments which fix the limits of the road
boundaries. Often, new road layouts must be approved by the community or its
representatives before it achieves legal status. This is always the case when a new
or revised layout requires the taking of private property.
