--- title: "Types of gradient and selection" subject: "Transportation Engineering I" chapter: 3 key: "semester_6/transportation_engineering_i:gradients-types" source: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/gradients-types site: BCT Engineering --- # Types of gradient and selection Transportation Engineering I · Chapter 3: Geometric Design of Highway - Asked in: 2077 Chaitra, 5 marks; 2075 Baisakh, 8 marks (asked 2 times) - Video: [Gradients Purpose and Factors Affecting the Selection of Gradient - Geometric Design of Railway](https://www.youtube.com/watch?v=-Xkp46iodPw) — Ekeeda - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/gradients-types - Whole chapter: https://www.bctengineer.com/md/semester_6/transportation_engineering_i/chapter-3 ## Question Discuss the different types of gradients used in highways and the factors to be considered in selecting them. ## Short answer Gradient types are ruling, limiting, exceptional, minimum, average, and momentum gradients. Selection depends on terrain, design speed, traffic volume and composition, construction and maintenance cost, vehicle characteristics, drainage, length of grade, horizontal curves requiring compensation, and safety. ## Answer **Gradient** (grade) is the rate of rise or fall of the road along its length, expressed as a percentage (or 1 in n). #### Types of gradient 1. **Ruling (design) gradient:** the maximum gradient used in the design of a road; heavy vehicles can climb it without loss of speed. IRC: plain/rolling 3.3% (1 in 30), mountainous and steep terrain 5%. 2. **Limiting gradient:** a steeper value used where the topography makes the ruling gradient costly. IRC: 5% (1 in 20) in plain/rolling terrain, 6% in hills. It should not be used over long continuous lengths. 3. **Exceptional gradient:** the steepest value allowed in short stretches (not more than 100 m) under unavoidable conditions. IRC: 6.7% (plain/rolling), 7% (hills). Nepal's hill roads allow higher values depending on the road class in NRS 2070. 4. **Minimum gradient:** the least grade needed for longitudinal drainage on kerbed roads, in cuts or on flat terrain: about 0.2% (1 in 500) for lined (concrete) drains and 0.5-1% (1 in 200 to 1 in 100) for unlined earth drains. 5. **Average gradient:** the ratio of the total rise or fall to the length of a section; used for assessing hill roads. 6. **Momentum gradient:** a steep grade following a downgrade, which a vehicle climbs using its momentum. (Figures follow IRC; refer to NRS 2070 for the exact table by road class and terrain.) #### Factors in selecting gradients 1. **Terrain** - plain, rolling, hilly, or mountainous. 2. **Design speed** and the class of road. 3. **Traffic volume and composition** - a high proportion of trucks needs flatter grades. 4. **Construction and maintenance cost** - flatter grades require more earthwork in hilly ground. 5. **Vehicle characteristics** - power-to-weight ratio, tractive resistance, brake performance. 6. **Drainage needs** and climatic conditions (snow, ice, rain). 7. **Length of grade** - long grades need a flatter gradient; provide level stretches for rest. 8. **Horizontal curves** - provide grade compensation. 9. **Safety:** sight distance on vertical curves, braking on long downgrades. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. The ruling (design) gradient is A. the steepest allowed for short stretches B. the minimum for drainage C. the maximum gradient used in design that heavy vehicles can climb without loss of speed D. the ratio of total rise to length Answer: C. Ruling gradient is the maximum design gradient that heavy vehicles can pull up without loss of speed. (Key point: Ruling gradient) 2. An exceptional gradient may be used A. throughout the road length B. only in short stretches (not more than 100 m) in unavoidable conditions C. only on plain terrain D. only for drainage Answer: B. Exceptional gradient is the steepest value, allowed only for short stretches in unavoidable situations. (Key point: Exceptional gradient) 3. Why is a minimum gradient provided? A. To reduce earthwork B. To help trucks climb C. To reduce speed D. For drainage on kerbed or flat roads Answer: D. Minimum gradient (0.5% lined, 1% unlined drains) ensures drainage on flat terrain. (Key point: Minimum gradient) 4. How does a high proportion of trucks affect gradient choice? A. It needs flatter grades B. It needs steeper grades C. It has no effect D. It removes the need for grade compensation Answer: A. Heavy vehicles have lower power-to-weight ratios, so flatter grades are required. 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