HGT Factors Affecting Highway Alignment

FACTORS AFFECTING HIGHWAY ALIGNMENT
Once the necessity of the highway is assessed, the next process is deciding the alignment. The
highway alignment can be either horizontal or vertical. The position or the layout of the central line
of the highway on the ground is called the alignment. Horizontal alignment includes straight and
curved paths. Vertical alignment includes level and gradients. Alignment decision is important
because a bad alignment will enhance the construction, maintenance and vehicle operating cost.
Once an alignment is fixed and constructed, it is not easy to change it due to increase in cost of
adjoining land and construction of costly structures by the roadside.
The requirements of an ideal alignment are:
 The alignment between two terminal stations should be short and as far as possible be
straight, but due to some practical considerations deviations may be needed.
 The alignment should be easy to construct and maintain. It should be easy for the operation
of vehicles. So to the maximum extend easy gradients and curves should be provided.
 It should be safe both from the construction and operating point of view especially at slopes,
embankments, and cutting. It should have safe geometric features.
 The alignment should be economical and it can be considered so only when the initial cost,
maintenance cost, and operating cost is minimum.
Factors controlling alignment:

We have seen the requirements of an alignment. But it is not always possible to satisfy all these
requirements. Hence we have to make a judicial choice by reconsidering all the factors. The various
factors that control the alignment are as follows:
 OBLIGATORY POINTS: These are the control points governing the highway alignment. These
points are classified into two categories. Points through which it should pass and points
through which it should not pass. Some of the examples are:
 Bridge site: The bridge can be located only where the river has straight and permanent
path and also where the abutment and pier can be strongly founded. The road approach
to the bridge should not be curved and skew crossing should be avoided as possible.
Thus to locate a bridge the highway alignment may be changed.
 Mountain: While the alignment passes through a mountain, the various alternatives are
to either construct a tunnel or to go round the hills. The suitability of the alternative
depends on factors like topography, site conditions and construction and operation cost.
 Intermediate town: The alignment may be slightly deviated to connect an intermediate
town or village nearby.
These were some of the obligatory points through which the alignment should pass. Coming
to the second category that is the points through which the alignment should not pass are:
 Religious places: These have been protected by the law from being acquired for any
purpose. Therefore, these points should be avoided while aligning.
 Very costly structures: Acquiring such structures means heavy compensation which
would result in an increase in initial cost. So the alignment may be deviated not to pass
through that point.
 Lakes/ponds etc: The presence of a lake or pond on the alignment path would also
necessitate deviation of the alignment.
 Traffic: The alignment should suit the traffic requirements. Based on the origin destination
data of the area, the desire lines should be drawn. The new alignment should be drawn
keeping in view the desire lines, traffic flow pattern etc.
 Geometric design: Geometric design factors such as gradient, radius of curve, sight distance
etc. also governs the alignment of the highway. To keep the radius of curve minimum, it may
be required to change the alignment of the highway. The alignments should be finalized
such that the obstructions to visibility do not restrict the minimum requirements of sight
distance. The design standards vary with the class of road and the terrain and accordingly
the highway should be aligned.
 Horizontal alignment: Horizontal alignment is one of the most important features

influencing the efficiency and safety of a highway. Horizontal alignment design involves the
understanding on the design aspects such as design speed and the effect of horizontal curve
on the vehicles. The horizontal curve design elements include design of super elevation,
extra widening at horizontal curves, design of transition curve, and set back distance.
 The design speed as noted earlier, is the single most important factor in the design of
horizontal alignment. The design speed also depends on the type of the road. For e.g.,
the design speed expected from a National highway will be much higher than a village
road, and hence the curve geometry will vary significantly.
 The presence of horizontal curve imparts centrifugal force which is reactive force acting
outward on a vehicle negotiating it. Centrifugal force depends on speed and radius of
the horizontal curve and is counteracted to a certain extent by transverse friction
between the tyre and pavement surface. On a curved road, this force tends to cause the
vehicle to overrun or to slide outward from the centre of road curvature. For proper
design of the curve, an understanding of the forces acting on a vehicle taking a
horizontal curve is necessary.
 Setback distance ‘m’ or the clearance distance is the distance required from the central
line of a horizontal curve to an obstruction on the inner side of the curve to provide
adequate sight distance at a horizontal curve.
 Vertical Alignment or Gradient: Gradient is the rate of rise or fall along the length of the
road with respect to the horizontal. While aligning a highway, the gradient is decided
designing the vertical curve. Before finalizing the gradients, the construction cost, vehicular
operation cost and the practical problems in the site also has to be considered. Usually steep
gradients are avoided as far as possible because of the difficulty to climb and increase in the
construction cost. More about gradients are discussed below.
The effect of long steep gradient on the vehicular speed is considerable. This is particularly
important in roads where the proportion of heavy vehicles is significant. Due to restrictive
sight distance at uphill gradients the speed of traffic is often controlled by these heavy
vehicles. As a result, not only the operating costs of the vehicles are increased, but also
capacity of the roads will have to be reduced. Further, due to high differential speed
between heavy and light vehicles, and between uphill and downhill gradients, accidents
abound in gradients.

HGT Factors Affecting Highway Alignment

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FACTORS AFFECTING HIGHWAY ALIGNMENT

The requirements of an ideal alignment are:

Factors controlling alignment:

 Horizontal alignment: Horizontal alignment is one of the most important features

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