6. Road Alignment: Highway Alignment
1) The position occupied by the center line of a road in plan is called road alignment.
2) The position or layout of the center line of highway on the ground is called the alignment.
3) Highway alignment includes both horizontal & vertical alignment of a roadway.
4) The horizontal alignment includes the straight path, the deviations and horizontal curves.
5) Changes in gradient & vertical curves are covered under vertical alignment of roads.
6) Improper alignment leads to more construction cost, maintenance cost, vehicle operation
cost & accident rate.
7) Basic requirements of alignment are short, easy, safe & economical.
7. Factors Controlling Highway Alignment:
1) Obligatory Points: obligatory points through which the road alignment has to pass are
generally due to the topographic & other site condition including natural obstruction.
Mountain pass, Bridge to cross a river, etc. There are obligatory points through which the
road should not pass like religious places, costly structure, unsuitable land, etc.
2) Traffic: Road alignment should be decided based on the requirements of road traffic.
Traffic flow, Traffic volume, Future trends, etc.
3) Geometric Design: Gradient, radius of curve, & sight distance also would govern the
final alignment of road..
4) Economics: While trying to decreasing the initial construction cost, either the future road
maintenance cost or vehicle operation cost or both of these may increase considerably.
Therefore, economic analysis should be done properly based on life cycle cost.
5) Other Consideration: others factors such as horizontal, vertical alignment, drainage
consideration, hydrological factors, political consideration & monotony.
8. Survey to be conducted for finalizing the Alignment:
1) Topographic Map Study: The process of making several possible alternative alignments of
a road on the topographical map of the area is known as map study.
2) Reconnaissance Survey: A general examination of a fairly broad stretch of land between
the terminal stations in the field, along the proposed alternative alignment marked on the
map is known as reconnaissance survey. Engineer visit the site & examine the general
characteristics of area. Rapid & rough survey between terminal station.
3) Preliminary Survey: The art of finding the details of alternative alignment found
suitable during the reconnaissance survey is known as preliminary survey. Field to office
work.
4) Detailed Survey or Location Survey: The detailed examination of the field along the
alignment finally recommend during the preliminary survey is called as location survey.
9. Road Intersection:
The places where two or more roads are arranged to join or cross at same or different levels
are known as road intersection or road junctions.
1) Intersection at Grade: All the roadways join or cross at the same level. Un-channelized &
channelized or rotary
2) Grade Separated Intersection: Separation in grade of the intersecting roads is achieved
by providing a bridge. Over bridge or under pass.
