---
title: "Highway curves and effects without superelevation"
subject: "Transportation Engineering I"
chapter: 3
key: "semester_6/transportation_engineering_i:curve-effects-without-super"
source: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/curve-effects-without-super
site: BCT Engineering
---

# Highway curves and effects without superelevation

Transportation Engineering I · Chapter 3: Geometric Design of Highway

- Asked in: 2070 Magh, 8 marks; 2068 Bhadra (old course), 8 marks (asked 2 times)
- Video: [Horizontal Curve || When No Superelevation is Provided || Transportation Engineering](https://www.youtube.com/watch?v=nKwTtr7sapE) — Shubham Mishra
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/curve-effects-without-super
- Whole chapter: https://www.bctengineer.com/md/semester_6/transportation_engineering_i/chapter-3

## Question

Explain the types of highway curves. What effects take place on a vehicle when it negotiates a horizontal curve without superelevation? Give reasons.

## Short answer

Without superelevation, the full centrifugal force must be resisted by friction, causing lateral skidding if v^2/(gR)>f, overturning if v^2/(gR)>b/(2h), load transfer, passenger discomfort, driver difficulty, and higher fuel and tyre wear, because no weight component opposes the outward force.

## Answer

#### Types of highway curves
**Horizontal curves**
- **Simple circular curve** - a single arc of constant radius joining two tangents.
- **Compound curve** - two or more arcs of different radii in the same direction with a common tangent at the junction.
- **Reverse curve** - two arcs bending in opposite directions.
- **Transition (spiral) curve** - varying radius between a tangent and a circular curve.

**Vertical curves**
- **Summit (crest) curve** - formed when an upgrade meets a downgrade or a lesser upgrade.
- **Valley (sag) curve** - when a downgrade meets an upgrade or lesser downgrade.

#### Effects on a vehicle on a horizontal curve without superelevation
When the pavement is flat (no superelevation), the whole centrifugal force $P = Wv^2/(gR)$ must be resisted by lateral friction alone.

1. **Lateral skidding:** if $v^2/(gR) > f$ the tyres slip sideways and the vehicle leaves the curve; a high speed or low $f$ (wet road) makes it likely.
2. **Overturning:** if $v^2/(gR) > b/(2h)$ the vehicle overturns about the outer wheels, a risk for high vehicles such as buses and trucks.
3. **Load transfer:** the outer wheels carry more load than the inner wheels, so the outer tyres wear more and the load shifts.
4. **Passenger discomfort:** passengers are thrown outward.
5. **Driver difficulty:** to prevent skidding the driver must slow down, so the speed and capacity decrease; or, if he does not, he runs on the outer lane and may meet opposing vehicles.
6. **Higher fuel use and tyre wear.**

**Reason:** the centrifugal force has no counterbalance except friction, and the friction available ($f = 0.15$ at design) is usually not enough for the design speed. A superelevation $e$ (and an adequate radius) provides a component of weight against $P$ so that $e + f = V^2/(127R)$ is satisfied.

## Check yourself

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

1. Which curve consists of two arcs bending in opposite directions?
   A. Compound curve
   B. Reverse curve
   C. Simple circular curve
   D. Transition curve
   Answer: B. A reverse curve has two arcs bending opposite ways; a compound curve has arcs in the same direction. (Key point: Types of horizontal curves)

2. A summit curve is formed when
   A. a downgrade meets an upgrade
   B. a tangent meets a circular arc
   C. an upgrade meets a downgrade or lesser upgrade
   D. two curves meet in opposite directions
   Answer: C. Summit (crest) curves occur where an upgrade meets a downgrade or lesser upgrade; valley curves are the opposite. (Key point: Vertical curves)

3. On a flat curve, what alone resists the centrifugal force?
   A. Lateral friction
   B. Weight component
   C. Camber
   D. Gradient
   Answer: A. With no superelevation the whole centrifugal force must be resisted by lateral friction. (Key point: Resisting force without superelevation)

4. What happens to wheel loads of a vehicle on an unsuperelevated curve?
   A. Inner wheels carry more load
   B. Load is shared equally
   C. Outer wheels carry more load, so outer tyres wear more
   D. Front wheels lose all load
   Answer: C. Centrifugal force transfers load to the outer wheels, causing more wear there and shifting loads. (Key point: Load transfer)

5. Why is superelevation needed in addition to friction?
   A. Friction (about 0.15) is usually not enough for design speed, so a weight component is needed
   B. Friction is always greater than needed
   C. Friction acts only on straights
   D. Superelevation reduces the radius
   Answer: A. Available friction is usually inadequate, so e is provided to satisfy e + f = V^2/(127R). (Key point: Reason)

## Related in Transportation Engineering I

- [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 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
- [Superelevation on 100 m radius curve](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3An-62j-super-100) — Transportation Engineering I, chapter 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×
- [Reasons for grade compensation](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Agrade-comp-reasons) — Transportation Engineering I, chapter 3, asked 3×

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