Roundabouts (Defined)

Insurance Institute for Highway Safety, Arlington, Virginia USA, reprinted with permission.

The modern roundabout is a circular intersection with design features that promote safe and efficient traffic flow. It was developed in the United Kingdom in the 1960s and now is widely used in many countries, including the United States, where its use is growing.

At roundabouts in the U.S., vehicles travel counterclockwise around a raised center island, with entering traffic yielding the right-of-way to circulating traffic. In urban settings, entering vehicles negotiate a curve sharp enough to slow speeds to about 15-20 mph; in rural settings, entering vehicles may be held to somewhat higher speeds (30-35 mph). As vehicles circulate within the roundabout, slow and consistent speeds are maintained by the deflection of traffic around the center island and the relatively tight radius of the roundabout and exit lanes.

Slow speeds help vehicles move smoothly into, around, and out of a roundabout. Drivers approaching a roundabout must reduce their speeds, look for potential conflicts with vehicles already in the circle and be prepared to stop for pedestrians and bicyclists. Once in the roundabout, drivers proceed to the exits they need.

Common traffic maneuvers at roundabouts

Right turn

Straight through

Left turn

Insurance Institute for Highway Safety, Arlington, Virginia USA, reprinted with permission.

Modern roundabouts are much smaller than older traffic circles — also known as rotaries — and require vehicles to negotiate a sharper curve to enter. As a result, travel speeds in roundabouts slower than speeds in traffic circles.

Because of the higher speeds in older traffic circles, many are equipped with traffic signals or stop signs to help reduce potential crashes. In addition, some older traffic circles and rotaries operate according to the traditional "yield-to-the-right" rule, with circulating traffic yielding to entering traffic.

Modern roundabout

Older traffic circle

Insurance Institute for Highway Safety, Arlington, Virginia USA, reprinted with permission.

The first modern roundabouts in the United States were constructed in Nevada in 1990. Since then, many more have been built, although the precise number is unknown. Roundabouts are much more common in some other countries, including Australia, the United Kingdom and France.

Although some states and cities have been slow to build roundabouts, they are gaining more popularity in the United States. Roundabouts are one of 20 evidence-based safety countermeasures recommended by the Federal Highway Administration (Federal Highway Administration, 2017).

Some states, such as New York and Virginia, have adopted "roundabout first" policies requiring that roundabouts be considered a preferred alternative when building new intersections or upgrading older ones if feasible (New York State Department of Transportation, 2011Virginia State Department of Transportation, 2009)

Roundabouts are appropriate at many intersections, including high-crash locations and intersections with large traffic delays, complex geometry (more than four approach roads, for example), frequent left-turn movements, and relatively balanced traffic flows. Roundabouts can be constructed along congested arterials and at freeway exits and entrances, in lieu of traffic signals.

Sometimes space constraints or topography make it impossible to build a roundabout. Geometric design details vary from one site to another and must take into account traffic volumes, land use, topography and other factors. Roundabouts often require more space in the immediate vicinity of the intersection than comparable traditional intersections. However, because roundabouts can reduce delays and queue lengths, they require less space on the approaching roads than comparable intersections controlled by stop signs or traffic signals.

An intersection with highly unbalanced traffic flows (that is, a very high traffic volume on the main street and very light traffic on the side street) may not be an ideal candidate for a roundabout. The same is true for isolated intersections in a network of traffic signals.

While the initial construction cost of a roundabout varies site by site, its maintenance usually is cheaper than for intersections with signals. The service life of a roundabout is significantly longer, approximately 25 years, compared with 10 years for a typical traffic signal (Rodegerdts et al., 2010).

Full Article
Roundabouts defined
Safety benefits
Safety challenges
Traffic flow benefits
Public opinion
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Reprinted with permission from the Insurance Institute for Highway Safety. Please visit here for the original article.

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