Alignment of the road: Introduction, Requirements, and factors

A highway alignment is the layout of the planned highway line on the ground or the position of the central line of the highway. 

Generally, highway alignment can be categorized into two types.

Horizontal Alignment: There are several types of horizontal roads, including straight, curved, and both. The alignment of the road is shown horizontally from the top view.

Vertical Alignment: Basically, this is concerned with gradients, slopes, and leveling. You can understand vertical alignment by looking at the road alignment from a perspective or from a front or side view.

Several disadvantages would arise from improper alignment: Increased construction costs, Increased maintenance costs, Increased vehicle operations costs, and Increased accident rates.

Changing the alignment of a road is difficult once it has been constructed and aligned due to the increased cost of adjoining land and the construction of costly structures. Therefore, let's review the important topics of highway alignment. Firstly, let's discuss the alignment requirements for highways.

Highway alignment requirements:

1. Easy: During construction, maintenance, and traffic operations, the alignment must be easy to maintain. To satisfy this requirement, the alignment should be easy for vehicles to operate with easy gradients and curves.

2. Short: Ideally, the alignment should be as short as possible between two terminal stations. A short alignment reduces construction and maintenance costs as well as transportation costs.
If this requirement is to be met, the alignment must be as straight as possible.

3. Economical: Alignments should be cost-effective in terms of construction, maintenance, and traffic operations.

4. Safe: It is important that the alignment is safe for traffic operation. As a result, the alignment needs to be stable enough from the viewpoint of hill slopes, embankments, cut slopes, and foundations of embankments in order for construction and maintenance to be possible.

The factors that affect road alignment

1. Obligatory points:

Highway alignment is governed by obligatory points. Alignment points determine where to pass and where not to pass. 

Road alignments to accommodate

The first category includes bridge sites, intermediate towns, mountain passes, etc.

The proposed road must pass through important towns, villages, or historical places, as shown in the figure. In this example, the straight result PQ will take the form of PRSQ to align the points R and Soon. For points, R and S that are of little significance, a straight line PQ with link roads RA and SB may be adopted.
In the road alignment, mandatory points must be accommodated.

Road alignments to avoid

Religious buildings, unsuitable land, and very expensive structures are all included in this category. The acquisition of a costly structure would result in heavy compensation, which would increase costs. It is generally unsuitable to use a site that is marshy or waterlogged.
On the alignment of the road, there are certain points that must be avoided.
Due to the special construction techniques required, the construction and maintenance costs are likely to be very high if there is no alternative.

2. Horizontal Curve and gradient:

It is important to have as flat a curve as possible. Depending on the minimum radius of the curve and the transition curves, adjustments may need to be made in the horizontal alignment of roads.

Whenever a new road is aligned, the gradient should be flat and lower than the ruling curve or design curve. Therefore, the alignment needs to be changed to avoid excessive falls or rises.

3. Sight Distance :

It is essential that the drivers have a minimum sight distance in order to avoid accidents.

Particularly during the night, aligning the road should not obstruct the visibility of drivers. As well, enough distance should be visible ahead for vehicles moving at the designed speed to safely overtake each other. As a result, the alignment should be finalized in a way that ensures good sight distance.

4. Railway Crossing:

Highway alignments should cross railway alignments at right angles, preferably.
Ideally, the alignment should cross the river or the railway line at right angles. If the crossing point is a river, the site should meet the essential requirements for a good bridge.
It is recommended to build a bridge over a rail lane to avoid traffic jams on fast-moving lanes, national highways, or expressways.

5. Earthwork :

Alignment should require less earthwork. Therefore, the alignment needs to be changed to prevent excessive cutting and filling. To avoid excessive cutting or filling, the alignment should be economical.

Leveling the land requires extensive earthwork and backfilling after land acquisition. Earthwork and backfilling have been deviated wherever necessary to avoid excessive cutting and backfilling.

6. Proper Drainage:

The alignment is designed to prevent water logging during monsoons by providing proper drainage at the edges.

7. Monotony:

In flat terrain, long straight paths can become monotonous for the driver, leading to accidents. As a result, it is important to provide suitable intervals between small horizontal curves to avoid monotony.

8. Economy:

The alignment should cost the least possible in terms of initial costs, operating costs, and maintenance costs. Whenever possible, avoid high embankments and deep cutting to reduce construction costs.
Road alignments should be chosen so that the construction, maintenance, and operation costs of the road are as low as possible. A complex structure should not be required or excessive cuttings & fillings should be avoided.

9. Formation bed:

Towards achieving this, it is acceptable to change the alignment just a little bit in order to go through the good soil that has sufficient strength.
As much as possible, the alignment should run on good soil. Therefore, alignment can be changed in order to satisfy this requirement.

10. The volume of traffic:

Traffic volume, annual traffic volume increase, and peak traffic are analyzed and alignment is based on all previous traffic data.

11. Availability of Labour and material:

Labor and materials for the road's construction should be close to the alignment. The alignment of the road is changed in order to make construction and maintenance of the road more economical.
Construction materials are a key factor in determining the quality of roads. During highway alignment planning, their availability needs to be taken into account.