---
title: "Setback on rural highway, 80 kmph"
subject: "Transportation Engineering I"
chapter: 3
key: "semester_6/transportation_engineering_i:n-71m-setback-80"
source: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/n-71m-setback-80
site: BCT Engineering
---

# Setback on rural highway, 80 kmph

Transportation Engineering I · Chapter 3: Geometric Design of Highway

- Asked in: 2071 Magh, 8 marks
- Video: [SET BACK DISTANCE - Highway Geometric Design - Transportation Engineering - GATE](https://www.youtube.com/watch?v=3M_1e89fnUU) — Ekeeda GATE & ESE
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/n-71m-setback-80
- Whole chapter: https://www.bctengineer.com/md/semester_6/transportation_engineering_i/chapter-3

## Question

Calculate the setback required from the edge of inner lane of pavement to an obstruction on a two lane rural highway designed at the speed of 80 kmph. Assume the length of circular curve as 150 m having ruling minimum radius and required sight distance as 200 m.

## Short answer

For two-lane highway at 80 km/h with ruling minimum radius 230 m and curve length 150 m, required setback from inner lane edge for 200 m sight distance is 18.53 m.

## Answer

#### Ruling minimum radius
With $e_{max}=0.07$ and $f_{max}=0.15$ (IRC / NRS 2070):

$$
R_{min}=\frac{V^2}{127(e+f)}=\frac{80^2}{127(0.22)}=229.1\ \text{m}
$$

Adopt **R = 230 m** (to the road centre line).

#### Setback
Assumptions: two-lane road, lane width 3.5 m, so the driver's path is at the centre of the inner lane, $n=1.75$ m from the centre line and the path radius is $R_i=230-1.75=228.25$ m. Sight distance $S=200$ m is greater than $L=150$ m, so the angle is based on the curve length:

$$
\frac{\alpha}{2}=\frac{L}{2R_i}=\frac{150}{2(228.25)}=0.32859\ \text{rad}=18.827^\circ
$$


$$
m_c=R_i\left(1-\cos\frac{\alpha}{2}\right)+\frac{S-L}{2}\sin\frac{\alpha}{2}=12.212+25\sin(18.827^\circ)=20.279\ \text{m}
$$

$m_c$ is measured from the centre of the inner lane. The setback from the edge of the inner lane (pavement edge) is

$$
m=m_c-\frac{w}{2}=20.279-1.75=18.53\ \text{m}
$$

(Measured from the road centre line this is 22.03 m.)

**Answer:** the obstruction must be kept at least **18.53 m** from the edge of the inner lane.

## Check yourself

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

1. How is R found for 80 km/h?
   A. R = V^2/(127(e+f)) with e = 0.07 and f = 0.15 = 229 m
   B. R = V^2/(127f) with f = 0.35
   C. R = V^2/225e
   D. R = 80 x 150
   Answer: A. Adopted as 230 m. (Key point: Ruling minimum radius)

2. Why is the S>L formula used?
   A. Because S is less than L
   B. Because S = 200 m exceeds L = 150 m
   C. Because R is small
   D. Because the road is two lane
   Answer: B. The angle is then based on L. (Key point: Case choice)

3. What does (S-L)/2 sin(α/2) account for?
   A. The superelevation
   B. The widening
   C. The straight parts of the sight line beyond the curve ends
   D. The lane width
   Answer: C. It adds 25 sin(18.83°) = 8.07 m. (Key point: Extra term)

4. How is the setback from the inner lane edge obtained?
   A. Add 1.75
   B. Use 20.279
   C. Subtract 3.5
   D. Subtract half the lane width: 20.279 - 1.75 = 18.53 m
   Answer: D. m_c is from the lane centre. (Key point: From centre to edge)

## Related in Transportation Engineering I

- [Setback on six lane highway, 550 m radius](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3An-72a-setback-350) — Transportation Engineering I, chapter 3
- [Setback on six lane highway, 450 m curve](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3An-73m-setback-six-lane) — 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
- [Setback on curve 400 m radius, 1.9 m](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3An-78p-setback-400) — Transportation Engineering I, chapter 3, asked 2×
- [Sight distance with obstruction on curve](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3An-78b-sd-obstruction) — Transportation Engineering I, chapter 3
- [Setback on six lane road, 500 m radius](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3An-65c-setback-500) — Transportation Engineering I, chapter 3

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