Features

Road Curve Design and Construction

Everything you need to know about road curves.

Updated on

August 12, 2023

What is a road curve?

Road curves are sections of the roadway that deviate from a straight line, either in an angular or gradual shift in the road’s direction.

The provision of a curve in a road adjusts to variations in the terrain and permits drivers to round a turn without having to stop suddenly or slow abruptly, which may avoid accidents and hence improve safety on the road.

Curves are classified into horizontal and vertical curves. Horizontal curves occur in the horizontal plane and are typically circular or parabolic.

In contrast, vertical curves occur in the vertical plane.

Types of horizontal road curves include simple curves, compound curves, deviation curves, reverse curves, and transition curves.

Types of vertical curves include valley curves and summit curves.

Here are the most popular road curves.

1. Horizontal curve

This type of curve enables the roadway to adapt itself to the natural contours of the terrain, a very important consideration particularly where steep or hilly topography exists.

Horizontal curves are designed in such a way that they would turn the road’s direction without allowing the vehicle’s speed to exceed the safe speed limit.

2. Simple curve

A simple road curve is just a single continuous bend of the road that gradually changes direction without sudden interruptions.

This is designed to enable drivers to negotiate a curve safely and comfortably by making a smooth transition from one direction to another without the road experiencing abrupt changes in its alignment.

3. Compound curve

This is a horizontal curve made up of two or more simple curves of different radii, which bend in the same direction and are on the same side of a common straight line.

This type of road curve makes a perfect transition from straight road portions to simple curves, thus enabling drivers to negotiate turns safely.

Compound curves are common in areas with restricted rights-of-way or rolling terrain where direct roadways are not feasible.

4. Transition curve

This is a curve in the road that gradually changes elevation on the outside of the curve to help drivers negotiate the turn smoothly at higher speeds.

It is a type of curve in the road that enables one to change curvature gradually and safely so that the driver will be able to maintain a comfortable speed while negotiating the curve.

Transition curves are common on high-speed roadways, with their length determined by road grade, radius of curvature, and design speed.

5. Reverse curve

A reverse curve is the opposite compared to a compound curve; it has two simple curves that bend on opposite sides of a common straight line.

This type of road curve is an attempt at maintaining the integrity of the road safety and the changes in speed, within a specified route, mainly where a compound curve is too complicated to navigate.

Reverse curves make smooth transitions of the road sections possible by using two simple curves that bend in opposite directions and join through a straight line.

This design reduces abrupt turns, thus helping the driver to follow the path more easily and safely at proper speeds.

6. Deviation curve

A deviation curve is a combination of two reverse curves.

This curve is often required to deviate from a given straight path to avoid intervening obstructions like buildings, bodies of water, or any other physical hindrances.

7. Superelevation curve

These are designed to prevent the lateral pressure that can throw a car out of balance while negotiating a curve.

Such road curves are banked to a certain degree, which aids vehicles in negotiations with safety at faster speeds.

8. Spiral curve

This curve smoothly transitions a road from straight to curved, with a constantly changing radius that allows vehicles to adjust speed without sudden changes.

These spiral curves ensure that the driver has a comfortable transition through the gradual adjustment of curvature, preventing sudden changes in directions and speeds.

These curves are extremely important on high-speed roads and highways where smooth traffic flow and comfort to the driver matter most.

9. Vertical curve

Vertical curves are used to transition from one gradient to another. These curves are used to manage the pace of slope change and to give drivers a smooth ride.

There are two types of vertical curves: valley curves and summit curves.

Valley curve – Also known as ‘sag curve’, valley curves are a type of curve that dips down and then rises back. These curves are placed at the base of hills.

This curve design allows vehicles to transit from a steeper uphill gradient to a flatter or downhill gradient gradually and safely, without going through sudden slope changes that may lead to potentially unsafe driving conditions.

Summit curve – Also referred to as ‘crest curve’, summit curves are a type of curve that rises and then dips down. These curves are placed at the peak of hills.

This curve design permits the vehicle to safely and progressively make a transition from a downhill slope to a flatter or uphill gradient without sharp changes in slope that may result in potentially unsafe driving conditions.