Alignment of Roads | Factors Controlling Highway Alignment


What Is Road Alignment?

Road alignment is the position of layout of the center line of the highway on the ground. In simple words, road alignment is the direction through which the road passes. Road alignment is also known as highway alignment.

There are two types of alignments of road. They are as follows:

  1. Horizontal alignment, and
  2. Vertical alignment.

1. Horizontal Alignment

Horizontal alignment consists of the following:

  • Straight path
  • Curves, and
  • Horizontal deviators

2. Vertical Alignment

Vertical alignment consists of the following:-

  • Vertical curves like summit curves and valley curves,
  • Vertical gradients.

The alignment of road is a very important aspect of road construction. It should be decided very carefully before the commencement of road construction. If the alignment of road is inappropriate then it may result in the following problems.

  1. Increased cost of construction of road.
  2. Increased cost of maintenance of road.
  3. Increase in operation cost of vehicle.
  4. Increased probability of accidents on the road.

So, keeping in mind the above issues, the alignment of roads should be decided very carefully before the commencement of its construction.

Requirements Of A Good Alignment

The must have properties of an ideal alignment are as follows:

1. Short

An ideal alignment of road should preferably be as short as possible, between two terminal stations. Straight alignments are usually the shortest ones. The major advantages of short alignment are that the construction cost of it is relatively cheaper and the travel time will be less, as the route will be the shortest.

2. Easy

An ideal alignment should be designed in such a way that its construction and maintenance should be uncomplicated. Also, it should be easy for the operation of vehicles with relatively easy curves and gradients.

3. Safe

An ideal alignment is designed in such a way that it follows the safety standards for the vehicle operating on that particular road. For example, steep curves should be avoided as gradual curves are safer for vehicles operating on the road. It minimizes the probability of road accidents.

5. Economical

While selecting the alignment of road, it should be kept in mind that the costs like its construction cost, maintenance cost, and operation cost are bought down to a minimum level.

Factors Controlling Alignment Of Road

The factors affecting the road alignment are as follows:

1. Availability Of Construction Materials

The materials required for the construction of roads should be available near the road construction site. If they are available nearby, it results in lowering the construction costs as well as the maintenance cost. It also reduces the transportation charges of materials.

2. Obligatory Points

Obligatory points are the controlling points that govern the highway alignments. The obligatory points are responsible for the deviation of highway.

The obligatory points are further divided into two categories as follows:

i) Points through which an alignment is allowed to pass – for eg. bridge site, important city, etc.

ii) Points through which an alignment should not pass. – for eg. religious places like temples and churches, any water body, monuments, valleys, etc.

3. Lower Grades And Easy Curves

The alignment selected for road should be such that low grades and easy curves are acquired. To acquire low grades and curves, sometimes it becomes necessary to increase the length of the road.

4. Crossing

The provision of crossings like steam crossing, river crossing, and railway crossing along the alignment of road should be studied thoroughly.

5. Geological Features

It should be kept in mind that the alignment of road should be made in such a way that no serious complexities are met from the geological features of the surrounding area, such as stability of the soil/ground, landslide, etc.

6. Proper Drainage

While making the alignment of road, a proper and sufficient drainage for water, especially the rainwater, should be provided along the alignment of road. Proper drainage is very important for the stability of road. Also, the factors like substance water level, seepage flow, and high flood level are to be considered.

7. Traffic

The alignment of road should be made in such a way that it should meet the traffic requirements. To know the trend of traffic, origin and destination study should be carried out. Also, the traffic flow patterns and traffic forecasting for the future should be studied before making the alignment of road.

8. Economy

The economy is one of the most important factors to keep in mind while deciding the alignment of the road. The costs like construction cost, maintenance cost, and operation cost are minimized up to a possible extent, without compromising the quality of work.

9. Miscellaneous Factors

– It should have proper visibility in all weather conditions and at any point of time throughout the day.

– It should not pass through forest areas and snowy areas.

– It should not pass through a densely populated area.

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