--- title: "Factors controlling highway alignment" subject: "Transportation Engineering I" chapter: 2 key: "semester_6/transportation_engineering_i:alignment-factors" source: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/alignment-factors site: BCT Engineering --- # Factors controlling highway alignment Transportation Engineering I · Chapter 2: Highway Alignment and Engineering Survey - Asked in: 2081 Chaitra, 5 marks; 2079 Chaitra, 8 marks; 2079 Ashwin, 6 marks; 2076 Bhadra; 2073 Bhadra; 2072 Magh; 2071 Bhadra, 8 marks; 2070 Magh; 2066 Magh (old course), 4 marks; 2064 Shrawan (old course); 2063 Kartik (old course), 4 marks; 2062 Jestha (old course), 8 marks (asked 12 times) - Video: [Alignment | Factors controlling highway alignment | Highway alignment](https://www.youtube.com/watch?v=RUr_rYgevCM) — Civil Engineer - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/alignment-factors - Whole chapter: https://www.bctengineer.com/md/semester_6/transportation_engineering_i/chapter-2 ## Question Explain the factors controlling (affecting the selection of) highway alignment. ## Short answer Highway alignment is controlled by obligatory points (must-pass and avoid), traffic demand, geometry standards (NRS), construction and operating cost, topography, hydrology, soil stability, material availability, environmental and social factors, and scope for future widening. ## Answer 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which is an obligatory point the road must pass through? A. A graveyard B. A mountain pass (saddle) or bridge site C. A reserved forest D. An airport Answer: B. Bridge sites, mountain passes and important towns are points the road must pass; graveyards, reserved forests, airports must be avoided. (Key point: Obligatory points) 2. Why is the shortest alignment not always the cheapest? A. Total cost = construction + maintenance + vehicle operation, which may be lower on a longer route B. Short roads cost nothing C. Length does not affect cost D. Cheaper routes are always longer Answer: A. Total cost includes construction, maintenance and operation, so the shortest route may involve costly earthwork or structures. (Key point: Shortest not always cheapest) 3. What should be avoided regarding geology and soil? A. Stable rock B. Good soil C. Landslide-prone, unstable, swelling clay and erodible zones D. Firm ground Answer: C. Unstable, landslide-prone, swelling clay and erodible zones should be avoided. (Key point: Geology and soil) 4. In hills, which slopes should be avoided because of climate? A. South-facing dry slopes B. Ridge tops C. Flat plains D. Shady north-facing wet slopes Answer: D. North-facing wet shady slopes are unstable and keep the road damp, so they are avoided. 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