---
title: "Raising outer edge for superelevation"
subject: "Transportation Engineering I"
chapter: 3
key: "semester_6/transportation_engineering_i:n-63k-super-raise"
source: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/n-63k-super-raise
site: BCT Engineering
---

# Raising outer edge for superelevation

Transportation Engineering I · Chapter 3: Geometric Design of Highway

- Asked in: 2063 Kartik (old course), 8 marks
- Video: [Superelevation | Superelevation in highway engineering](https://www.youtube.com/watch?v=9B7QslOHeN8) — 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/n-63k-super-raise
- Whole chapter: https://www.bctengineer.com/md/semester_6/transportation_engineering_i/chapter-3

## Question

How much should be the outer edges of the pavement to be raised with respect to the centre line on a two lane road designed for mix traffic at a speed of 80 km/hr on a horizontal curve of radius 200 m if the super elevation is obtained by rotating the pavement with respect to the (i) centre line, and (ii) inner edge.

## Short answer

With e=0.07 and total width 7.8 m, outer edge raised by 0.27 m (centre-line rotation) or 0.55 m (inner-edge rotation).

## Answer

#### Superelevation
$V=80$ km/h, $R=200$ m, mixed traffic.
- IRC (75% of speed, no friction): $e=\dfrac{V^2}{225R}=\dfrac{80^2}{225(200)}=0.142$, which exceeds the maximum 0.07 (IRC/NRS 2070 for plain/rolling terrain). Provide **e = 0.07**.
- Friction needed: $f=\dfrac{V^2}{127R}-e=0.252-0.07=0.182$, which is more than 0.15. A speed of 80 km/h is therefore not fully safe on this curve. The safe speed with $e=0.07$ is $\sqrt{127(200)(0.22)}=74.8$ km/h, so speed should be restricted to about 75 km/h (or $R\ge229$ m used).

The raising of the edge is worked out for $e=0.07$.

#### Width to be raised
Two lanes, $W=7.0$ m, extra widening ($l=6.1$ m): $W_m=\dfrac{2(6.1)^2}{2(200)}=0.186$ m, $W_{ps}=\dfrac{80}{9.5\sqrt{200}}=0.595$ m, $W_e\approx0.8$ m. Total width $B=W+W_e=7.8$ m.

#### (i) Rotation about the centre line
The outer edge rises and the inner edge falls by the same amount relative to the centre line:

$$
h_{outer}=\frac{eB}{2}=\frac{0.07(7.8)}{2}=0.273\ \text{m}
$$

The inner edge is lowered by 0.273 m below the centre line.

#### (ii) Rotation about the inner edge
The inner edge stays at its level and the outer edge is raised above it by the full amount:

$$
h_{outer}=eB=0.07(7.8)=0.546\ \text{m}
$$

The centre line is raised by 0.273 m.

(The camber is neglected here. In practice the outer half is first rotated flat, which adds the camber drop, and then rotated to the full superelevation.)

**Answer:** outer edge raised by about **0.27 m** (centre-line rotation) or **0.55 m** (inner-edge rotation) relative to the reference line.

## Check yourself

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

1. e = V^2/225R = 0.142. What is used and why?
   A. 0.07, the maximum permitted
   B. 0.142
   C. 0.15
   D. 0.10
   Answer: A. e is capped at 0.07 (friction needed then exceeds 0.15, so speed should be restricted). (Key point: Superelevation adopted)

2. What is the outer-edge rise for rotation about the centre line?
   A. e B = 0.546 m
   B. e B / 2 = 0.273 m
   C. e B / 4
   D. e W only = 0.49 m
   Answer: B. Each edge moves half the total difference, eB/2. (Key point: Centre-line rotation)

3. What is the outer-edge rise for rotation about the inner edge?
   A. e B = 0.546 m
   B. e B / 2 = 0.273 m
   C. 2 e B
   D. e B / 4
   Answer: A. The inner edge is fixed, so the outer edge rises the full amount. (Key point: Inner-edge rotation)

## Related in Transportation Engineering I

- [Transition curve and edge elevations, hilly terrain](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3An-79a-tc-super-elevations) — Transportation Engineering I, chapter 3
- [Transition curve about centre line, 65 kmph](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3An-67m-tc-65) — Transportation Engineering I, chapter 3
- [Setback on 400 m curve, 1.95 m lane distance](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3An-68m-setback-1-95) — Transportation Engineering I, chapter 3, asked 2×
- [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
- [Transition curve 220 m, pavement 7 m](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3An-77c-tc-220) — 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

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