What is a Roundabout ?

Roundabouts were developed in England in 1956. Several decades of technological advances have greatly improved performance and led many road agencies to adopt them as an alternative to traffic signals.  Roundabouts can have a variety of designs, but all true roundabouts have the following features:

  • Circular Roadway:  1-way counterclockwise flow around a center island allows traffic from all directions to safely use the roundabout at the same time.
  • Yield at Entry:  Traffic entering the roundabout yields to traffic circulating within the roundabout.
  • Deflection:  Traffic is redirected from a straight path in order to circulate around the central island, which results in low speeds and safe operation.
  • Precise Geometry:  The size and shape of the roundabout is designed specifically to meet the traffic patterns and physical constraints of the particular intersection.

Roundabout in Oskemos (photo courtesy of Dave Sonnenberg, Ingham County Road Commission)

Roundabout in Okemos, Michigan
(photo courtesy of Dave Sonnenberg,
Ingham County Road Commission)

 

 

 

 

 

 

 

 

Roundabouts are not the best solution at every intersection, but in many cases they provide better traffic control than signals. Safety, reduced delay, attractive appearance, and speed control are the primary reasons that roundabouts are becoming more popular. Crash rates are generally 40-60% less than a signalized intersection and injury crashes are 35-80% less. The crashes that do occur at a roundabout are much less serious and rarely fatal. The improved safety results from eliminating broadside and head-on collisions, reducing traffic speeds through the intersection and providing pedestrians a safety refuge when crossing. Roundabouts usually cause less delay than traffic signals and can eliminate the need to widen narrow roads.

In spite of extensive roundabout utilization around the world, they have only recently been implemented in the U.S. People in this country are often unfamiliar with roundabouts and skeptical about whether they will work. However, opinion surveys in U.S. cities where roundabouts have been constructed have shown an 80-90% approval rating. In the past ten years, over 100 modern roundabouts have been constructed in many states, including Indiana.

Roundabouts are often confused with traffic circles, which are viewed negatively because of congestion, high crash rates, poor entry conditions and high speeds. However, there are significant differences between traffic circles and roundabouts. The main differences are shown in the following table:

Traffic Circles

Roundabouts

Diameter is arbitrarily determined by the space available and is often very large, allowing higher speeds and unsafe conditions.

Diameter is calculated based on traffic volumes and are much smaller.

Approaches are generally perpendicular to the traffic circle (requires drivers to stop and turn right at a 90 angle), or they are designed to permit a high speed merge.

Approaches are flared and deflected around the central island and speeds are constrained to 10 - 20 mph by design. Entering vehicles must yield to circulating vehicles.

High speeds do not allow adequate gaps, for merging traffic and can cause a complete stoppage of traffic within the circle. Speeds can approach 50 mph.

Low speed environment (20-25 mph) creates adequate gaps. Traffic within the roundabout has right-of-way, preventing entering traffic from causing traffic stoppages within the roundabout.

Monument Circle in Indianapolis is an example of a traffic circle (although speeds on this circle rarely hit 50 mph). Traffic circles can be used successfully in low traffic situations, such as within a subdivision.


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