Why Are Roads Built With So Many Curves?

Published Categorized as Infrastructure
Road Curves

Roads have been around since the ancient Romans started building them within their cities and conquered lands. Since then, roads have developed into more complex engineering structures connecting different cities, islands, and even countries.

To build such a network of roads in various places with varying altitudes, slopes, and obstacles, engineers need to divert from designing straight roads and apply curves as necessary.

Roads are built with so many curves to prevent cutting by changing the alignment of the road. It avoids obstructions of a straight road and makes gradients more comfortable. Road curves avoid construction over expensive land and make road travel stable in places with steep grades.

Even though a straight road is more economical since the cost of materials and labor is reduced, curved roads are necessary for turns, comfort, and safety. In this article, we’ll discuss the science behind the curves found in our modern roads.

How Curves Work on Roads and Track Alignment

Curves work on roads and even railways to shift the track alignment or change the direction of the path. Here are some of the functions of road curves you can refer to:

  • They’re built for the safety and comfort of the passengers riding on a public transport vehicle.
  • They’re built for hassle-free turning and changing directions.
  • To provide a gradual change in the orientation of the alignment.

Factors That Affect Road Alignment

Road alignment is the position of the centerline of a road on a construction plan. Several factors require a change of alignment and use of road curves. 

These include:

  • Presence of gradient or slope in a terrain. No regular vehicle can travel on a steep road. So, to reduce the steepness of the road, curves are usually applied on the road.
  • Obstruction along the way. Ponds, lakes, rivers, buildings, and enormous rocks are all considered obstructions in constructing roads. Road curves are used to avoid an expensive cost of materials and labor over these obstacles.
  • Requires a slow traffic flow. Traffic flow varies depending on the area, and usually, straight roads require fast traffic flow. Wherein curved roads require slow traffic flow.
  • Lengthy or costly straight route. It doesn’t mean that the road is straight, as it’ll travel shorter to the next point or station. There are cases that a straight road will be a longer route to travel. And a long way means more expensive construction. So, to avoid this problem, curves are built on roads.
  • Aesthetical reason. A change of alignment or road curves is applied for the passengers to enjoy a city or region’s beautiful sight and scenery.

Types of Curves Built on Roads

Let’s now take a closer look at the types of curves built on the roads.

Horizontal Curves

This type of curve is laid out in a horizontal construction plan. It has different kinds that serve a diverse purpose depending on its construction requirements.

Simple Curve

This type of curve is characterized by the degree of the curve or by the length of the radius. It’s called a simple curve because it only contains a single radius length, unlike other curves with multiple radius lengths.

Simple curves are constructed on roads to change the route or railroads alignment, commonly seen in plains or sloped areas.

For railroads, a minimum radius length must be constructed for all travelers passing by safety and convenience. The radius length should be no less than 574.15 ft (175 m) for broad gauge. 

Having a higher radius length than this will make the travel longer and construction costs expensive. While making it lower than this will make it either bumpy or hazardous to all passengers.

Compound Curve

This kind of curve contains two or more simple curves with varying radius lengths. It’s called a compound curve because of having multiple curves on the road in the same direction.

Compound curves are built on roads to avoid obstacles along the way that can’t be evaded using a simple curve. Its two adjacent bends usually have a common tangent line that connects them.

Reverse Curve

This is a type of horizontal curve that contains two simple curves in opposite directions. It may have an equal or varying radius length on the curve.

Reverse curves are applied on roads requiring a frequent change in the direction of travel, especially in hilly areas or large rocks scattered in a region. This type of curve is also required for crossovers in station posts.

Transition Curve

This type of curve contains different radius lengths that vary slowly from infinity to a finite value. The finite values in the transition curve must be equivalent to that of a connected circular curve.

Transition curves are constructed on roads to create a slow and relaxed change of direction to avoid a relatively large area of obstruction, land, or hills. Its curve design considers two factors primarily affected by the terrain to be used.

These include:

  • The passengers’ discomfort shouldn’t be affected by the rate of change of centrifugal acceleration.
  • The passengers’ discomfort shouldn’t be affected by the rate of superelevation.

Transition curve designs usually vary in plain or rolling terrain and mountainous or steep terrain.

Vertical Curves

This type of curve provides ease in rising or falling gradients on roads, which include two main kinds of vertical curves listed below. Vertical curves are used on roads to give all passengers enough comfort, provide a slow change in grade or slope while traveling, and deliver sufficient traffic flow visibility.

Summit Curve

This type of vertical curve is used in a convex curve road. Such examples are roads on top of hills and mountains.

The general idea of using a summit curve is that it always starts in a rising gradient, and it only varies in the next connecting road, whether it meets a falling gradient, horizontal gradient, or another rising gradient with a different slope.

Valley or Sag Curve

This type of vertical curve is used in concave curve roads. So, the concept of this type of curve is that it always starts in a falling gradient, and it only varies in the next connecting road, whether it meets a rising gradient, horizontal gradient, or another falling gradient with a different slope.

The primary difference between a summit curve and a sag curve is the starting point. It’s called a summit curve because passengers will start from a rising slope or gradient.

While a sag curve is the direct opposite of a summit curve, meaning that the passengers will begin from a falling slope or gradient.

Final Thoughts

Roads have developed into a complex engineering structure, while some of them are considered man-made wonders today. Many facilities were added through the years to provide comfort to passengers, including road curves.

Curves on roads serve many purposes why it’s necessary to build them. Aside from changing the direction or alignment of the straight road, curves are used to avoid obstructions along the way and provide a gradual turn while traveling along the curve.

This is just an added feature in constructing road networks, but it has made traveling safer and easier like never before.


By Giovanni Valle

Giovanni Valle is a licensed architect and LEED-accredited professional and is certified by the National Council of Architectural Registration Boards (NCARB). He is the author and managing editor of various digital publications, including BuilderSpace, Your Own Architect, and Interiors Place.

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