9 Factors Controlling Highway Alignment

Highway alignment is the positioning of the centerline of the highway. 9 factors controlling highway alignment will be discussed in this article.

Highway alignment is also called road alignment.

Simply, it is the direction through which the road will pass.

Highway alignment provides proper guidance to pass the road through the path which is most economical, easy to construct, and free from conflicts.

There are two types of highway alignment.

a. Horizontal Alignment

It includes straight stretches, curves, etc.

b. Vertical Alignment

It includes upward or downward gradients.

1. Class & Purpose

The alignment of the road is affected by class and purpose.

National and state highways between two stations must be aligned straight as much as possible whereas, in the case of other types of roads, a deviation may be allowed where it is found necessary.

2. Obligatory Points

Obligatory points define the way through which road should be passed and through which road should not be passed.

The roads are usually built for the development of the areas. Therefore, road alignment must necessarily pass through important towns, groups of villages, and places of religious, social, political, and commercial importance.

Road alignment should not be passed through historical places, cultural places, etc.

3. Type of Vehicular Traffic

In the case of fast-moving traffic, the road alignment must be straight as far as possible.

In the case of low traffic, the alignment may even have sharp turns.

4. Gradient

The alignment of roads should be selected such that longitudinal slopes are not steeper than the ruling gradient. To achieve this, the alignment might need a deviation from the straight line.

5. Horizontal Curves

In the case of national and state highways, the radius of the horizontal curve must not be less than 230 m otherwise the alignment must be improved.

6. Sight Distance

The alignment of roads must be decided such that more and clear sight distance is available for drivers of the vehicles.

7. Obstructions

Obstruction also affects alignment selection. For example, the alignment should be improved to avoid marshy land, ponds, wells, graveyards, historical, monumental, and religious sites, etc.

8. Economical Factor 

Road construction becomes economical when the road alignment is straight. So, straight alignments should be given priority.

9. Availability of Construction Materials

Construction material should be easily available. The use of abundant materials leads to a low construction cost.