Road alignment is the positioning of the centerline of the highway or road. We use alignment to show where that road will be constructed. There are 2 alignments to every alignment of the road. There is horizontal alignment and vertical alignment.
Alignment may be defined by a series of points, lines and curves. In order to construct a road, you need to know exactly where it is situated on the earth. Each point has given coordinate northing and easting that defines its address 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 exam city where that curve is located on the earth. Each line the nd curve must connect exactly on a shared point, or be coincident. Otherwise, you have an error in your alignment. The alignment is described by stationing, which introduces to the distance horizontally between the stationing points. The beginning stationing of alignment is defined by previous designs. The designer built the stationing different for each alignment so that it doesn’t get confuse the builder.
Typically, the alignment passes 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 the profile view.
The existing ground prior to construction is below the new roadway on this plan. This vertical alignment may be defined by points, lines and curves. The stationing of this alignment compares the horizontal alignment. The stationing is actually not shown in the attached picture. It is at the bottom of the sheet. In this case, the driver starts out moving downhill for a little bit and then enter a vertical curve and start moving uphill.
The basic principles of road alignment should be as follows:
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 serves 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 Affecting 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 as 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 be passed through historical places, cultural places, etc. The roads are usually built for the development of the areas. Their 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 given according to the type of vehicular traffic. In the case of fast-moving traffic, the road alignment must be as straight as much as possible whereas, in the case of low traffics and small vehicles, bullock carts, tongas, etc, the alignment may even have sharp turns.
The alignment of roads should be selected so 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
The alignment of roads should be given with flat curves wherever needed. In the case of national and state highways, the radius of the horizontal curve must not be less than 230 m in order to have less radius otherwise the alignment must be improved.
6. Sight Distance
The alignment of roads must be decided such that more 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.