Factors controlling highway alignment

The alignment of a highway is the route (centre line) selected on the ground. Its selection depends on many factors, which can be grouped as follows.

1. Obligatory points (control points)

• Points the road must pass through: bridge sites, mountain passes (saddles), important towns or existing roads.
• Points the road must avoid: religious and historic places, graveyards, costly buildings, reserved forests, airports, lakes and unstable land.

2. Traffic

• The alignment should serve the origin-destination pattern, linking the main traffic generators.
• Traffic volume and composition decide the design standard.

3. Geometric design

• Design speed, gradient, curves, sight distance and widths from NRS 2070 (or IRC) limit what is possible.
• Avoid steep grades and sharp curves; keep the sight distance adequate.

4. Economy

• Total cost = construction + maintenance + vehicle operation. The cheapest alignment is not always the shortest. Balance cut and fill and keep earthwork, structures and land acquisition low.

5. Topography

• Avoid very rough, steep or hummocky ground. In hills, follow the contours within the ruling gradient; in plains, take nearly straight routes.

6. Hydrology and drainage

• Consider the number and size of rivers and streams; cross streams at right angles at narrow, stable places; avoid flood-prone, marshy and high-water-table land.

7. Geology and soil

• Prefer stable rock or good soil; avoid landslide, unstable, swelling-clay and highly erodible zones.

8. Materials

• Availability of construction materials (aggregate, sand, water) near the road.

9. Climate and environment

• Rainfall, snow, wind, fog; avoid shady, wet north-facing slopes in hills; avoid forests and wildlife areas; meet EIA requirements.

10. Political and social factors

• Land acquisition, resettlement, administrative boundaries, defence and security.

11. Future expansion

• Possibility of widening and upgrading later.

A good alignment is one that satisfies most of these factors at the least total cost.