Alignment of road: Factors affecting- obligatory points with figures

Alignment of road is very important during the road construction (for exams and interviews too). We will sweep through what is highway alignment and then the requirements of highway alignment. Followed by that, I will walk you through factors affecting alignment of road. Then, I will describe obligatory points in depth with figures. In nutshell, l am going to make you savvy in the topic.

What is highway alignment?

The position or layout of center line of the highway on the ground is called alignment of road. It includes straight paths, horizontal deviation and curves. Improper alignment would result in one or more of the following disadvantages.

1. Increase in construction cost
2. Increase in maintenance cost
3. Increase in vehicle operation cost
4. Increase in accident rate

Once the road is aligned and constructed, it is not easy to change the alignment due to increase in cost of adjoining land and construction of costly structures. Therefore, let’s take a trip through the important topics under highway alignment. Let’s start from requirements of highway alignment.

Basic requirements of highway alignment

There are 4 basic requirements for the alignment of road.

1. Short– Length will be short means, it is straight, reduces the time of travel
2. Easy– Easy to construct and maintain the road with minimum problems and easy for operation of vehicles.
3. Safe– The alignment should be safe enough for construction and maintenance from the viewpoint of stability of natural hill slopes, embankment and cut slopes etc. also it should be safe for traffic operations.
4. Economical– Total cost including initial maintenance and vehicle operation cost should be minimum

Now, you know the requirements of highway alignment. Let’s dig in deep through the factors affecting alignment of road.

Factors affecting alignment of road

For an alignment to be shortest, it should be straight. This is not always possible due to practical difficulties such as intermediate obstructions and topography. The ideal condition rarely exist and it becomes necessary to adjust the highway alignment as per prevailing conditions.

The various factors affecting alignment of road are,

1. Obligatory points

The controlling points which govern the highway alignment are known as obligatory points. They are mainly responsible for the deviation of highway from its straight location.

For the purpose of convenience the obligatory points are divided into two.

1. Obligatory points which are to be accommodated on alignment of road
2. Obligatory Points which are to be avoided on alignment of road

The figure below shows the deviated alignment adopted due to river. The actual road was supposed to be through road PQ. But the suitable bridge site is not available along the road alignment because of peculiar characteristics of river or stream.

1. Obligatory points which are to be accommodated on alignment of road

There are various examples for first category, bridge site, intermediate town, mountain pass etc.

As shown in figure below, the proposed road has to pass through intermediate important towns, villages, or places of historical importance. Here, the straight result PQ will take the shape of PRSQ to put the points R and Soon the alignment. If points R and S are not much significance, the straight line PQ may be adopted with link roads RA and SB.

2. Obligatory points which are to be avoided on alignment of road

It includes religious places very costly structures, unsuitable land etc. Acquiring costly structures would mean heavy compensation resulting in increased cost. Marshy and waterlogged are generally unsuitable.

However if there is no alternative and the alignment has to be taken across such an area, the construction and maintenance costs are likely to be very high due to special construction technique.

That’s it about the first factor. Let’s look into other important factors affecting alignment

Also read: Classification of roads-5 types of roads full details

2.Type of vehicular traffic- a determining factor for alignment of road

The alignment should be provided according to the type of vehicular traffic. In case of fast moving traffic, the road alignment should be as straight as possible.

3. Geometric  design factors- an important factor for alignment of road

Geometric design factors such as gradient, radius of curve, sight distance etc. would govern the final alignment of highway

4. Availability of road building materials

The materials for construction of road are a determining factor. Their availability should be ensured while planning on the highway alignment.

5. Geological factors

The highway alignment should be made in such a way that no serious difficulties are met from geological factors of the locality such as stability of earth work, land slides etc.

6. Class and purpose

National highway and State highways between two stations should be aligned as straight as possible where as in the case of other types of roads, deviation may be permitted.

7. Earthwork

To avoid excessive cutting or filling, the alignment should be economical.

8. Public demand

The selected alignment of the road should meet the public demand of the area.

MUST READ: Road margins- 6 types of road margin in highway

So, you have read through all the vital details about alignment of road. What is that I missed out? Now, its time for you to tell me. Also, drop your doubts in the comments.

Enjoy learning!