Highway Alignment, it’s requirements & affecting factors for selection


The position or the layout of the center line of the highway on the ground is called the Highway Alignment.

Horizontal alignment of the road includes straight path, horizontal deviation and curves.

Vertical alignment of the road includes changges in the gradient and vertical curves.

Fig : Road

Disadvantages of Improper Alignment

a. Increase in construction cost.

b. Maintenance cost increasing.

c. Inceasing of Vehicle Operation Cost.

d. Rise in accident rate.

Requirements of road alignment (SESE)

The basic requirements of an ideal alignment of road are :

a. Short

The road alignment should be designed in such that the distance between two terminal stations is very less.
Straight alignment is the shortest but there may be several practical considerations which may cause deviation from the shortest path.

b. Easy

The alignment should be easy to construct and maintain the road with minimum problems.

Besides; the alignment should be easy for the operation of the vehicles with easy gradients and curves.

c. Safe

Alignment should be safe enough from the landslide, rockfalls, etc which makes easy in construction, maintenance and operation.

Also it should have best geometrical features causing safety for high traffic operation.

d. Economical

The road alignment is considered to be economical if the total cost including initial cost, maintenance cost and vehicle operation cost is lowest.

The alignment should be such that it would offer maximum utility by serving maximum populations and products.

Factors Controlling Highway Alignment

Following are the various factors which control the selection of Highway Alignment:

a. Obligatory Points

Obligatory points are the control points that governs the selection of highway alignment.

These points are divided into two categories:

1.Points Through which the alignment should pass

Points through which the road alignment should pass may cause the alignment to obtain deviate from the shortest and easiest part. These points include places like bridge site, intermediate town, a mountain pass or a quarry.

2. Point through which the alignment shouldn’t pass

These points includes religious places , very costly structures, un-suitable land (like marshy , peaty, water logged area, lakes, ponds, valleys) , etc.

b. Traffic

Origin and destination study should be carried out in the area and the desired line should be drawn ( Curve / Graph drawing) showing the trend of traffic flow.

The alignment and structure should be designed on the basis of studied traffic details and also assuming other future conditions and requirements.

c. Geometrical Design

Geometrical design factors such as gradients , radius of curve, sight distance , design speed, maximum allowable super elevation,etc. Geometrical design also governs the selection of final alignment of highway.

Design of the highway should be designed such that it provides optimum efficiency in traffic operations and maximum safety at a resonable cost.

d. Economical

Road alignment should be economical. It becomes economical when it is short and safe.

e. Other Considerations

Various other factors which may govern the selection of alignment are drainage considerations, hydrological factors, political considerations and monotony.

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