---
title: "Reasons for extra widening"
subject: "Transportation Engineering I"
chapter: 3
key: "semester_6/transportation_engineering_i:extra-widening-reasons"
source: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/extra-widening-reasons
site: BCT Engineering
---

# Reasons for extra widening

Transportation Engineering I · Chapter 3: Geometric Design of Highway

- Asked in: 2081 Ashwin, 3 marks; 2070 Bhadra; 2068 Bhadra (old course), 4 marks (asked 3 times)
- Video: [Extra widening on road | Extra widening on curve | HIGHWAY ENGINEERING](https://www.youtube.com/watch?v=jAm1n-bYmxY) — 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/extra-widening-reasons
- Whole chapter: https://www.bctengineer.com/md/semester_6/transportation_engineering_i/chapter-3

## Question

State the reasons for introducing extra widening of the carriageway on a horizontal curve. (Write a short note on extra widening.)

## Short answer

Extra widening on curves compensates for off-tracking (rear wheels follow smaller radius), front overhang swing, driver difficulty maintaining lane, need for safer passing clearance, and higher speed steering. Formula: W_e = nl²/(2R) + V/(9.5√R). Provided when R < 300 m and introduced gradually along transition curve.

## Answer

**Extra widening** is the additional carriageway width given on a horizontal curve beyond the normal width of the straight section.

#### Reasons
1. **Off-tracking:** the rear wheels follow a smaller radius path than the front wheels, so a long vehicle needs more width (mechanical widening).
2. **Wider swing of the front end:** the front overhang sweeps a larger arc outside the track of the front wheels.
3. **Driver's difficulty:** on curves drivers find it harder to keep to the lane; they tend to drive away from the pavement edges and need extra clearance (psychological widening).
4. **Vehicle crossing:** a safer clearance between two vehicles passing or overtaking on a curve is needed, as lateral clearances become uneven with speed.
5. **Higher speed vehicles** need more width for steering.

The extra width is $W_e = \dfrac{n l^2}{2R} + \dfrac{V}{9.5\sqrt{R}}$ and is generally provided when $R < 300$ m; it is introduced gradually along the transition curve.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Why does off-tracking need extra width?
   A. Rear wheels follow a smaller radius path than the front wheels
   B. Front wheels are heavier
   C. The road is cambered
   D. Speed is lower
   Answer: A. Rear wheels track inside the front wheels' path, so the vehicle occupies more width. (Key point: Off-tracking)

2. Psychological widening is provided because:
   A. Vehicles are heavy
   B. Road is long
   C. Drivers keep greater clearance from edges and find lane keeping harder on curves
   D. Pavement is rough
   Answer: C. Drivers drive away from edges on curves. (Key point: Psychological)

3. Extra widening is generally provided when:
   A. R is more than 1000 m
   B. R < 300 m
   C. Always on straights
   D. Only on bridges
   Answer: B. For larger radius it is negligible. (Key point: When provided)

4. Extra widening is introduced:
   A. Suddenly at the curve start
   B. Only at the middle
   C. Gradually along the transition curve
   D. Only on shoulders
   Answer: C. It is introduced gradually along the transition length. (Key point: Introduction)

## Related in Transportation Engineering I

- [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×
- [Advantages of shoulder and extra widening](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Ashoulder-extra-widening-adv) — Transportation Engineering I, chapter 3
- [Effect of centrifugal force on horizontal curve](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Acentrifugal-effect) — 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
- [Setback on four lane carriageway, 400 m radius](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3An-70bh-setback-four-lane) — Transportation Engineering I, chapter 3