Highway roads
design involves application of grading, surface texture, and sight distance
concepts.
These concepts
are vital in providing road facilities which are safe, efficient, comfortable,
economical and structurally sound.
Road design is
controlled by the following factors
- Reduction of environmental pollution in terms of noise and air pollution.
- Reduction of number and severity of accidents road safety measure
- Traffic volume and Directional distribution.
- The maximum safe speed that can be maintained over a section of road when design or ideal or favourable operation condition exists.
- Topography and land use.
The various elements of highway location and design are;
·
Determination
of horizontal and vertical alignment, with attention to obtaining proper sight
distance and super elevation.
·
The design of
roadway cross sections, intersections, ramps, and service roads.
In this article
sight distance overview elements are discussed.
Sight Distance
A primary feature in the design of any roadway is the availability
of adequate sight
distance for the driver to make decisions while driving.
The various sight distances include:-
Stopping Sight Distance.
Stopping sight distance is the distance ahead
that a motorist should be able to see so that the vehicle can be brought safely
to a stop on sighting an obstruction or foreign object in the road. This
distance include the driver’s reaction or perception distance and the distance
traveled while the brakes are being applied. The distance traveled varies with
respect to initial speed, the brake reaction time, and the coefficient of
friction for wet pavements.
1. Perception reaction
time (lag time)
Time between sighting of the hazard and the first application of
brakes
Dependent on driver’s alertness, distance of
obstruction, and speed
2. Braking distance
Distance the vehicle moves from first application of brakes to
stop.
Usually dependent on the condition of the tyre, roads, vehicle
speed etc.
Decision Sight Distance. Stopping sight distances are usually
sufficient to allow reasonably competent drivers to come to a hurried stop
under ordinary circumstances.
However, these distances may not be sufficient for drivers when
information is difficult to perceive, or when unexpected maneuvers are
required.
The following are examples of locations where it is desirable to
provide decision sight distance:
(1) Exit ramps,
(2) Diverging roadway
terminals,
(3) Intersection stop bars,
(4) Changes in cross section, such as toll plazas and lane drops
Passing Sight Distance. Refer to Minimum distance on a two lane two
way carriage way that a driver should have ahead of him to safely overtake a
slow moving vehicle in front of him without interfering with the speed and
safety of an oncoming vehicle traffic in the
passing zone.
Superelevation
and Curves
Whenever design speed
exceed 70 mile/h or Centerline deflection of 0°4, it is necessary to introduce a
horizontal curve to assist the driver. Curves are usually accompanied by super
elevation, (a banking of the roadway to help counteract the effect of
centrifugal force on the vehicle as it moves through the curve). In addition to
super elevation, centrifugal force is also offset by the side friction
developed between the tires of the vehicle and the pavement surface..
A maximum rate of 0.06 is recommended for urban high-speed
roadways (50 mile/h or greater), while 0.04 is used on low-speed urban roadways
and temporary roads. Horizontal
curvature can be looked in terms of degree of curve as well as radius.
Under this definition, the degree of curve is defined as
the central angle of a 100-ft arc using a fixed radius.
Vertical Curves. A vertical curve can be technically be
defined as a parabolic curve centered on the intersection point of the vertical
tangents and is used to provide a smooth transition between vertical tangents
of different grades.
One of the principles of parabolic curves is that the rate of
change of slope is a constant throughout the curve.
For a vertical curve, this
rate is equal to the length of the curve divided by the algebraic difference of
the grades. This value is called the K value and represents the distance
required for the vertical tangent to change by 1 percent. The K value
help to determine
the minimum length of vertical curve necessary to provide minimum
stopping sight distance given two vertical grades.
More about curves will be discussed on the next article.
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