Introduction to Modern Roundabouts

What is a Modern Roundabout?

A modern roundabout is a circular, unsignalized intersection where entering traffic yields to motorists circulating counterclockwise around a central island. Modern roundabouts are engineered to maximize safety and reduce overall delay at an intersection.

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Where are Modern Roundabouts Located?

Although relatively new to the United States, modern roundabouts can be found in both warm and cold weather regions, including Alaska. The first modern roundabout in Michigan was constructed in 1996 by the Road Commission for Oakland County at the intersection of Tienken, Washington, and Runyon Roads in Rochester Hills. Today, modern roundabouts can be found at 16 locations statewide, with more locations slated for constructed in 2007.

Modern Roundabout Terminology

Central Island

A raised area in the center of the roundabout that traffic circulates around.

Circulating Road

The curved path used by vehicles to travel in a counterclockwise movement around the central island.

Entry

The point where vehicles enter the roundabout.

Exit

The point where vehicles exit the roundabout from the circulating road.

Inscribed Circle Diameter

The basic parameter used to define the size of a roundabout. It is measured between the outer edges of the circulating road.

Pedestrian Crossing

The location where pedestrians and some non-motorized vehicles cross the roadway utilizing the splitter islands.

Splitter Island

A raised or painted area on an approach used to separate entering from exiting traffic, deflect and slow entering traffic, and provide refuge space for pedestrians crossing the road.

Yield Line

A pavement marking used to mark the point of entry into the circulating roadway which is generally marked along the inscribed circle.

The Washtenaw County Road Commission would like to thank DLZ Michigan, Inc. for supplying this information.

History of the Modern Roundabout

In 1904, Columbus Circle located in New York City was the first circulating roadway, or traffic circle, built in the United States. The operating rules for traffic circles were not clearly defined and were confusing to motorists in the early part of the century. At Columbus Circle, The north-south traffic had priority or right-of-way over east-west traffic. Some U.S. courts decided that the “first-in” rule, or circulating traffic yields to entering traffic, would be the most practical solution to entry control at traffic circles.

By 1929, some engineers pointed out that the main drawback to the “first-in” rule was that circulating traffic locked during peak hours of the day when volumes were higher. To counteract this effect, they recommended changing the entry control to the current day “yield-to-left” rule, where circulating traffic has the right-of-way over entering traffic. However, these advocates were unable to convince the traffic engineering community to implement such a change.

From the 1930s onward, in conjunction with rapidly developing automobile technology, traffic engineers began designing larger traffic circles to accommodate higher speeds through an intersection. The entry control for these new designs was similar to common day interstate freeway interchange ramps in urban areas where weaving maneuvers are required to enter and exit the roadway. Since distances between the entrance and exit points were increased, gridlock within the circle was minimized. These new designs became known as rotaries. Rotaries are found primarily in the northeastern part of the United States. As populations and traffic volumes increased in these regions, rotaries also began locking up, making them inefficient. In the 1950s, traffic circles and rotaries fell out of favor in the United States largely because of capacity problems. In many cases, road authorities began installing traffic signals at these locations to promote traffic progression.

In the 1950s, British traffic engineers began questioning the American practice of rotaries, arguing that long weaving sections, combined with higher speeds made possible by larger diameter circles, were detrimental to traffic capacities and safety. During that decade, the modern day roundabout was born in the U.K. Designs changed from larger diameter circles with emphasis on merging and weaving to smaller central islands where the driver’s task was to accept a gap in the circulating flow. In 1966, the United Kingdom Department of Transportation adopted the “priority rule” (same as the yield to left principle here in North America) as the official entry control for modern roundabouts. By reducing the size of the central island, bringing the yield line closer to the center of the intersection, and widening the entries to the roundabout, officials documented that the level of service at modern roundabouts increased by 10 to 50 percent when compared to traffic circles and rotaries in the U.S.

Beginning in the early 1990s, Maryland and New York Departments of Transportation began implementing the U.K. design of modern roundabouts to provide higher levels of safety and traffic capacity at problem intersection locations. Since then, many more State DOTs and local road authorities have followed their lead by converting signalized and all-way stop controlled intersections with modern roundabouts.

The Road Commission would like to thank the creators of the Alaska Roundabouts website for supplying this information.