Modern roundabout

A modern roundabout is a junction where traffic goes one way round a central island and has priority over the traffic coming in.¹ Its arms have splitter islands, which keep entering traffic apart from leaving traffic, and deflect and slow it.^1,2,3 The curves keep speeds on the ring typically under 50 km/h (30 mph), and as all traffic goes round the same way, there are no left-turn or head-on conflicts.^2,3

Circles came first: one-way traffic began round Columbus Circle in New York in 1904, and round the Place de l'Etoile in Paris in 1907.⁴ Entering traffic often had priority, and as traffic grew, circles locked up; in America, with their crashes and congestion, they fell out of favour from the 1950s.^2,4

The modern roundabout was developed in the United Kingdom.² Tests by the Road Research Laboratory found that giving priority to the traffic on the roundabout raised capacity by 10 percent over no control, police control or signals; injury accidents fell by 40 percent.⁴ The rule became mandatory in November 1966, and roundabouts were made smaller: the driver's task became to take a gap in the traffic on the ring, not to weave.^2,4 Roundabouts reached Australia and some communities in France in the 1970s, and France had about 15,000 by mid-1997.⁴

The first two in the United States were built in Summerlin, Nevada, in March 1990.⁴ Studies of American junctions converted from traffic signals or stop signs, mostly of single-lane roundabouts, found 72 to 80 percent fewer injury crashes. At two-lane roundabouts crashes fell less, or even rose, though their safety improves over time as drivers get used to them.⁵