The geometric design of roads is the branch of highway engineering concerned
with the positioning of the physical elements of the roadway according to
standards and constraints. The basic objectives in geometric design are to
optimize efficiency and safety while minimizing cost and environmental
damage. Geometric design also affects an emerging fifth objective called
"livability," which is defined as designing roads to foster broader
community goals, including providing access to employment, schools,
businesses and residences, accommodate a range of travel modes such as walking,
bicycling, transit, and automobiles, and minimizing fuel use, emissions and
environmental damage. Geometric roadway design can be broken
into three main parts: alignment, profile, and cross-section. Combined,
they provide a three-dimensional layout for a roadway.
The alignment is the route of the road, defined as a series of horizontal
tangents and curves. The profile is the vertical aspect of
the road, including crest and sag curves, and the straight grade lines
connecting them. The cross section shows the position and
number of vehicle and bicycle lanes and sidewalks, along with their cross slope
or banking. Cross sections also show drainage features, pavement structure
and other items outside the category of geometric design.
Design standards Roads are designed in conjunction with
design guidelines and standards. These are adopted by state and national
authorities. Design guidelines take into account speed, vehicle type, road grade,
view obstructions, and stopping distance. With proper application of
guidelines, along with good engineering judgement, an engineer can design a
roadway that is comfortable, safe, and appealing to the eye.
The primary US guidance is found in A Policy on Geometric Design of Highways
and Streets published by the American Association of State Highway and
Transportation Officials. Other standards include the Australian Guide
to Road Design, and the British Design Manual for Roads. An open source version
of the green book is published online by The Council for Scientific and
Industrial Research office in Zimbabwe. Profile
The profile of a road consists of road slopes, called grades, connected by
parabolic vertical curves. Vertical curves are used to provide a gradual
change from one road slope to another, so that vehicles may smoothly navigate
grade changes as they travel. Sag vertical curves are those that have
a tangent slope at the end of the curve that is higher than that of the
beginning of the curve. When driving on a road, a sag curve would appear as a
valley, with the vehicle first going downhill before reaching the bottom of
the curve and continuing uphill or level.
Crest vertical curves are those that have a tangent slope at the end of the
curve that is lower than that of the beginning of the curve. When driving on
a crest curve, the road appears as a hill, with the vehicle first going
uphill before reaching the top of the curve and continuing downhill.
The profile also affects road drainage. Very flat roads and sag curves may have
poor drainage, and steep roads have high velocity flows.
= Terminology= = Sag Curves=
Sag vertical curves are curves which, when viewed from the side, are concave
upwards. This includes vertical curves at valley bottoms, but it also includes
locations where an uphill grade becomes steeper, or a downhill grade becomes
less steep. The most important design criterion for
these curves is headlight sight distance. When a driver is driving on a
sag curve at night, the sight distance is limited by the higher grade in front
of the vehicle. This distance must be long enough that the driver can see any
obstruction on the road and stop the vehicle within the headlight sight
distance. The headlight sight distance is determined by the angle of the
headlight and angle of the tangent slope at the end of the curve. By first
finding the headlight sight distance and then solving for the curve length in
each of the equations below, the correct curve length can be determined. If the
SL curve length is smaller than the headlight sight distance, then this
number can be used. If it is larger, this value cannot be used.
These equations assume that the headlights are 600 millimetres above the
ground, and the headlight beam diverges 1 degree above the longitudinal axis of
the vehicle. = Crest Curves=
Crest vertical curves are curves which, when viewed from the side, are convex
upwards. This includes vertical curves at hill crests, but it also includes
locations where an uphill grade becomes less steep, or a downhill grade becomes
steeper. The most important design criterion for
these curves is stopping sight distance. This is the distance a driver can see
over the crest of the curve. If the driver cannot see any obstructions in
the roadway, such as a stalled vehicle or animal, the driver may not be able to
stop the vehicle in time to avoid a crash. The desired stopping sight
distance is determined by the speed of traffic on a road. By first finding the
stopping sight distance and then solving for the curve length in each of the
equations below, the correct curve length can be determined. The proper
equation depends on whether the vertical curve is shorter or longer than the
available sight distance. Normally, both equations are solved, then the results
are compared to the curve length. Sight Distance > Curve Length
Sight Distance