There are a number of factors which are to be considered while selecting the alignment of roads. These factors also vary according to the type of roads and the areas through which these roads pass.
The factors which control alignment of roads in plain areas as under:
1. Class & Purpose
The alignment of the road may be decided keeping in mind the class and purpose of the road. National and state highways between two stations should be aligned as straight as possible whereas in the case of other types of roads, deviation may be permitted where found necessary.
2. Obligatory Points
The roads are basically constructed for the development of the areas. Their alignments should 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 provided according to the type of vehicular traffic. In case of fast moving traffic, the road alignment should be as straight as possible whereas in case of bullock carts, tongas, etc, the alignment may even have sharp bends.
The alignment of roads should be selected so that longitudinal slopes are not steeper than ruling gradient. To achieve this, the alignment might need a deviation from straight line.
5. Horizontal Curves
The alignment of roads should be provided with flat curves wherever found necessary. In case of national and state highways, the radius of horizontal curve should not be less than 230 m. in order to have minimum radius the alignment must be changed.
6. Sight Distance
The alignment of roads should be decided such that minimum sight distance is available for drivers of the vehicles.
The alignment of roads is decided keeping in mind the obstruction. For example, the alignment should be changed to avoid marshy land, ponds, wells, graveyards, historical monumental and religious sites etc.
8. Cross Drainage Works
The alignment should be adjusted so that minimum numbers of cross drainage work are required. To achieve this, the alignment should be chosen along the water shed line if possible.
While crossing river, the sites should provide all the requirements needed for a good bridge construction. The alignment of the road should cross the river at 900 to avoid construction of skew bridge.
10. Railway Crossing
A road alignment should cross a railway preferably at 900.
11. Formation Bed
The selected alignment should be through such areas which provide good formation bed.
12. Earth Work
To avoid excessive cutting and filling, the alignment should be changed where found necessary.
13. Agricultural Land and Dense Forests
The alignment of roads should be decided so that costly agricultural land and dense forests are avoided for the acquisition of the land.
14. Existing Right of Way (ROW)
In case upgrading a road, its existing right of way should be utilized for curtailing the cost of land acquisition.
15. Public Demand
The selected alignment of the road should meet the public demand of the area.