Road alignment is the positioning of the centerline of the highway or road. It is also called as highway alignment. Simply, it is the direction from which the road will pass. Road alignment provides a proper guidance to pass the road through the alignment which is most economical, easy to construct and free from conflicts. There are two types of alignment.
a. Horizontal Alignment – Straight stretches, curves
b. Vertical Alignment – Upward or downward gradients
Improper road alignment leads to the following:
a. Increase in the rates of accidents.
b. Increase construction and maintenance cost.
c. Increase the vehicle operating cost and also leads to the discomfort of the users.
d. The decrease in durability and strength.
Alignment is defined by a series of points, lines and curves. To build a road, you need to know exactly where it is located on the earth. Each point has given coordinate northing and easting that defines its location on the earth. Each line has a bearing and distance. Each curve has a radius and length and several points that have coordinates that describe exactly where that curve is located on the earth.
Each line and curve must connect exactly on a shared point, or be coincident. Otherwise, you have an error in your alignment. The alignment is defined by stationing, which refers to the distance horizontally between the defining points. The beginning stationing of alignment is defined by previous designs, or by the designer. The designer tries to make the stationing different for each alignment so that it doesn’t confuse the builder.
Typically, the alignment follows the centerline of the road. If you are driving down a road that has centerline striping, that yellow line(s) in the centre of the road probably closely follows the horizontal alignment.
The vertical alignment follows the horizontal alignment but is referring to whether or not you are going up a hill, or are on the crest of a curve or sag (low point). Below is a picture of vertical alignment in profile view.
The existing ground before construction is below the new roadway on this plan. This vertical alignment is also defined by points, lines and curves. The stationing of this alignment matches the horizontal alignment. The stationing is not shown in the attached picture. It is at the bottom of the sheet. In this profile, the driver would start out going downhill for a little bit and then enter a vertical curve and start going uphill.
Basic principles of road alignment
1. The alignment of the road should be as short as possible to give the economy in the cost of construction.
2. The alignment should be as straight as much as possible which provides higher speed to the traffic and lower cost of construction.
3. The road alignment must be easy for the construction, maintenance and traffic operation.
4. The alignment should cross the railway lines and other roads and bridges at right angles.
5. It should cross the rivers, canals or streams etc., at a place where its width is minimum.
6. The alignment should serve the maximum population by connecting intermediate important towns and a group of villages.
7. The road alignment should not pass through regions of natural beauty and scenery.
8. The alignment should be such that it crosses the minimum number of bridges, crossing culverts and embankment places.
9. It must give a smooth curve and easy gradient.
10. It should be such that minimum earthworks in embankment or cutting are done.
11. The alignment should provide good sight distance.
12. It should be free from obstruction like ponds, lakes, wells, monumental buildings and historical buildings etc.
13. The alignment should run through such places where materials of road construction and labour are easily available.
14. As far as possible it should run on good soil having the good bearing capacity to bear loads of traffic safely without any damage to the road.
15. The alignment should not passes through more costly and cultivated land. It must also avoid forests.
Factor Controlling Road Alignment:
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 pass and through which road should not be passed. Road alignment should not be passed through historical places, cultural places, etc. The roads are usually built for the development of the areas. Therefore, road alignments must necessarily pass through important towns, group of villages and places of religious, social, political and commercial importance.
3. Type of Vehicular Traffic
The alignment should be selected according to the type of vehicular traffic. In the case of fast-moving traffic, the road alignment must be straight as much as possible. In the case of low traffic, the alignment may even have sharp turns.
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.
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
The 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. Abundant material leads to higher cost of the material. This directly affects the economical condition.
Steps in Road Alignment
There are four major steps in road alignment. They are:
a. Map Study
The first step of road alignment is map study. Generally, geographical and contour maps are studied. This helps to know the position of natural features like ponds, rivers, lakes, hills etc. This also helps to learn about different alternative alignments.
b. Reconnaissance Survey
Reconnaissance is a rough survey done by visiting the field. Positions of natural features in the field are cross-checked as indicated in the map. If some features are not indicated in the map then such features are included. It also includes the collection of data like several cross drainage structures, high flood level, soil characteristics, etc. Report of selected alignment is also prepared that includes the merits, demerits and profile map of scale 1:50,000.
c. Preliminary Survey
The main objectives of the survey are:
1. To analyze the different alternatives to the road alignment.
2. To estimate the cutting and filling required in the construction.
3. To survey the alignments proposed by reconnaissance survey.
4. To finalize the alignment among different alternatives.
d. Detailed Survey
It is the final step before starting the construction work. In this detailed study of the following is done:
1. Soil Study
2. Hydrological Survey
3. Construction cost and time
4. Return on investment
5. Present and Future Traffic