Let’s take a closer look at these simple, yet complex circular mobility solutions called roundabouts.
You’ve seen them. The circular traffic-calming features popping up on roadways all over the United States. They can cause confusion to the uninitiated, but have been proven to reduce the severity of collisions. They do this by reducing the speed with which motorists enter the intersection as they travel around a central island.
Roundabouts were first developed from circular junction intersections, much like the Place de l'Étoile around the Arc de Triomphe in Paris. The first modern version of a roundabout was opened in 1899 in Germany. Since then, a number of iterations of the popular intersection have been developed. Roundabouts are more common in Europe than they are in the United States. Half of all of the world’s roundabouts are in France.
To better understand the differences in this ever-present transportation feature, SEH Alternative Intersection/Interchange Designer Scott Hotchkin, PE, explains a variety of designs.
First, let’s take a look at the anatomy of a roundabout.
A roundabout is a circular intersection designed to keep traffic flowing smoothly in one direction around a central point. Roundabout design is an iterative process that requires a good balance of the geometric design parameters including the inscribed diameter, entry radius and the exit radius. These parameters geometrically control speeds and allow for a smooth traffic flow. Traffic volumes, pedestrian volumes, design vehicle, surrounding property and approach speeds all influence the design.
Now that we better understand the different parts of a roundabout, let’s look at some of the more common versions.
Conventional roundabouts take up a lot of space. Therefore, they can’t be used in every circumstance. Enter the mini-roundabout. These smaller roundabouts have a mountable central island, making for a smaller footprint. This allows larger trucks to drive over the central island while still encouraging most vehicles to navigate on the circulatory road surface. This design is a good option for slower speed tight urban settings.
As their name suggests, this form of roundabout contains a singular lane circling around the central island. This is the safest and most basic form of the roundabout intersection style.
Also as its name would suggest, multi-lane roundabouts have more than one lane. These are the most common form of roundabouts in the United States. Typically, the inside lane is used to make left turns and through movements while the outside lane is used to make the second through movement and also the right turn.
A turbo-roundabout is a relatively new type of roundabout with a spiraling flow of traffic, requiring drivers to choose their direction before entering the roundabout. This type of roundabout is believed to be a safer and more efficient alternative to the standard multi-lane roundabout.
A typical roundabout has narrow curb-to-curb widths and tight turning radii, which are not ideal for large and oversized vehicles. An OSOW roundabout is designed to accommodate larger vehicles with wider truck aprons with a minimum slope and mountable curbs. The designs are larger overall. Roundabouts have design flexibility making them a viable alternative at nearly any intersection regardless of design vehicle size.
A roundabout interchange typically occurs between a highway or freeway and a minor road. Drivers enter the roundabout after exiting the highway, and before the adjoining the minor road. These roundabouts are typically grade-separated from the highway and create an easy transition between the different roadways. These interchanges are usually more cost effective than a traditional signalized interchange due to the smaller bridge width required for roundabouts.
A roundabout crossover interchange follows the same general principle as a diverging diamond interchange (DDI). Instead of using signalized ramp terminal intersections, it uses roundabouts. While a traditional DDI does not allow through movement from the ramps or a fifth or sixth leg, the roundabout crossover interchange does. This type of interchange works well with heavy turns to and from the highway. Much like a traditional DDI. Both the roundabout crossover interchange and the DDI handle traffic more efficiently than a traditional diamond interchange.
A roundabout single point urban interchange moves all ramp and minor road movements into one intersection, in this case a roundabout. This interchange has various forms, including with the minor roads positioned either over or under the highway. To equalize the spacing between the minor street intersections near the highway, roundabout SPUIs can be offset.
When implemented correctly, the circle is a simple geometric shape that can be used for complex mobility solutions. One lane, two lanes, three. The possibilities are endless to keep people moving in the direction they want to go.
Roundabouts may not be the best solution at every site, but any alternative intersection that significantly reduces major injury/fatality accidents should always be a design consideration.
Scott Hotchkin, PE is a roadway designer who has designed over 140 unique and effective roundabouts.