--- title: "Reasons for grade compensation" subject: "Transportation Engineering I" chapter: 3 key: "semester_6/transportation_engineering_i:grade-comp-reasons" source: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/grade-comp-reasons site: BCT Engineering --- # Reasons for grade compensation Transportation Engineering I · Chapter 3: Geometric Design of Highway - Asked in: 2077 Chaitra, 3 marks; 2065 Kartik (old course), 6 marks; 2063 Kartik (old course), 4 marks (asked 3 times) - Video: [Grade compensation in road | Grade compensation in highway | Highway Engineering](https://www.youtube.com/watch?v=0DJiLfDMgT8) — Engineer's Coach - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/grade-comp-reasons - Whole chapter: https://www.bctengineer.com/md/semester_6/transportation_engineering_i/chapter-3 ## Question What are the causes/reasons for providing grade compensation in highways (horizontal curves)? Give three reasons. ## Short answer Grade compensation reduces gradient on sharp curves because curve resistance adds extra tractive resistance, so a reduced grade equalizes total resistance and lets vehicles maintain uniform speed safely. IRC gives compensation as (30+R)/R %, max 75/R %, and none is needed for gradients flatter than 4%. ## Answer Grade compensation is the reduction of the gradient on a sharp horizontal curve. The reasons are: 1. **Curve resistance:** on a curve the vehicle meets extra tractive resistance due to lateral friction, tyre slip and steering. If the full ruling gradient also exists there, the total resistance may be more than the engine can overcome, causing loss of speed or stalling. 2. **Equal total resistance:** the gradient is reduced by an amount equal to the curve resistance, so that the total tractive effort needed on the curve is the same as on the straight with the ruling gradient. 3. **Safety and uniform speed:** vehicles keep a uniform speed, driving is easier and the accident risk is reduced, especially for heavy trucks. 4. **Hill roads:** where hairpin bends and sharp curves lie on long steep grades, heavy vehicles cannot climb without compensation. IRC rule: compensation $= \dfrac{30+R}{R}\%$, not exceeding $\dfrac{75}{R}\%$ (R in m); it is not needed for gradients flatter than 4%. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is grade compensation on a horizontal curve? A. Increasing the gradient on a curve to help drainage B. Reducing the gradient on a sharp horizontal curve C. Widening the pavement on a curve D. Raising the outer edge of the curve Answer: B. Grade compensation is the reduction of the ruling gradient on a sharp curve to offset the extra curve resistance. (Key point: Meaning of grade compensation) 2. Why does a vehicle on a sharp curve need more tractive effort than on a straight? A. Because of extra resistance from lateral friction, tyre slip and steering B. Because the road is always wetter on curves C. Because the vehicle weight increases on curves D. Because air resistance disappears on curves Answer: A. Lateral friction, tyre slip and steering add curve resistance, which with the full gradient may exceed engine capacity. (Key point: Curve resistance) 3. By how much is the gradient reduced on the curve? A. By the amount of the camber B. By the amount of superelevation C. By an amount equal to the curve resistance, so total tractive effort stays the same as on the straight D. By half the ruling gradient Answer: C. The reduction equals the curve resistance so total resistance equals that on a straight with the ruling gradient. (Key point: Equal total resistance) 4. According to the IRC rule given, compensation is not needed for gradients flatter than A. 2% B. 6% C. 10% D. 4% Answer: D. Gradients flatter than 4% need no grade compensation; above that the (30+R)/R % rule applies, capped at 75/R %. (Key point: IRC rule) ## Related in Transportation Engineering I - [Curve resistance and grade compensation](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Acurve-resistance-grade-comp) — Transportation Engineering I, chapter 3, asked 3× - [Extra widening: reasons and derivation](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Aextra-widening-derive) — Transportation Engineering I, chapter 3, asked 8× - [Highway curves and effects without superelevation](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Acurve-effects-without-super) — Transportation Engineering I, chapter 3, asked 2× - [Vertical curve with horizontal curve, 45 degrees](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3An-77c-vc-horizontal) — Transportation Engineering I, chapter 3 - [Reasons for extra widening](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Aextra-widening-reasons) — Transportation Engineering I, chapter 3, asked 3× - [Tangents, circular and transition curves](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Ah-alignment-elements-use) — Transportation Engineering I, chapter 3 ## Related in other subjects - [Report on road accidents on highways](https://www.bctengineer.com/question.md?key=semester_5%2Fcommunication_english%3Areport-road-accidents-highways) — Communication English, chapter 9 - [Compensation capacity for 15 MW, 33 kV load](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Acompensation-capacity-15mw-33kv) — Power System Analysis I, chapter 6 - [Reasons for I/O interfaces](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_organization_and_architecture%3Aio-interface-reasons) — Computer Organization and Architecture, chapter 7 - [EAC from given power-angle curves](https://www.bctengineer.com/question.md?key=semester_5%2Fpower_system_analysis_ii%3Apower-angle-curves-eac) — Power System Analysis II, chapter 6