Roundabout Capacity

Roundabouts can be categorized as follows:

  • Low-capacity roundabouts have one-lane entries.

  • Medium-capacity roundabouts have at least one two-lane entry.

  • High-capacity roundabouts have at least one three-lane or four-lane entry.

The earliest roundabouts are presented by year of construction in the following table. The number of lanes on the widest entry is given for each roundabout.

North America's Earliest Nine High-Capacity Roundabouts

Year Opened


No. of Lanes on Widest Entry

1990 South Roundabout, Sumerlin/Las Vegas, NV 3
1993 Long Beach Roundabout, Long Beach, CA 4
1994 Banbury Cross Roundabout, Summerlin /Las Vegas, NV 3
1995 Hualapai Roundabout, Summerlin/Las Vegas, NV 4
1995 I-70/Vail Road South Roundabout, Vail, CO 3
1996 Addison Roundabout, Addison, TX 3
1997 I-70/Avon Road North Roundabout, Avon, CO 3
1997 Avon Road/Beaver Creek Road, Avon, CO 3
2004 M-53/Van Dyke Ave/18 1/2 Mile Road, Sterling Heights, MI 3


The United Kingdom has hundreds of high-capacity roundabouts. In North America, since we have many more high-flow intersections than the United Kingdom, we need many more high-capacity roundabouts.

The highest concentration of traffic crashes tend to be at signalized intersections. By replacing signalized intersections with roundabouts, crashes could be redueced by nearly 50%. See Safety Benefits for more on this topic.

Roundabouts should be considered at high-speed, rural intersections because they significantly slow down traffic. In the United Kingdom it is the intersection of choice where high-speed highways cross. Traffic signals that regulate high-speed crossings tend to cause too many angle and rear-end crashes, often with serious injuries. The most important high-speed crossings to convert to roundabouts are those carrying high flows.

In North America our sites of worst delay and longest queues are often high-flow signalized intersections. We have many signalized intersections from which traffic backs up during peak periods as far as the eye can see. High-capacity roundabouts ordinarily eliminate long queues when they replace traffic signals.

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