The terminology surrounding circular roadway intersections can be confusing – traffic circle? roundabout? rotary? – but traffic engineers have embraced them in recent years as a way to allow traffic to flow through intersections with fewer accidents and less delay.
The idea of making cars go in circles when roads cross each other, rather than stopping and taking turns, was developed at the dawn of the automotive era, but it fell out of favor in the U.S. by the 1960s.
New designs – most importantly, shrinking the circle and giving right of way always to traffic in the circle – have brought it back to life.
A 2014 report from the New Hampshire Department of Transportation listed 36 roundabouts that have been built in the state in the previous decade, and more have been built since then. A group called Roundabouts USA estimates that 4,800 existed in the U.S. as of the end of 2015.
Roundabouts are safer than intersections because they eliminate the most dangerous type of collision: the broadside or T-bone accident, in which one vehicle slams into the side of another at a right angle, usually when one vehicle has run a red light or stop sign.
Accidents that do happen in a rotary tend to involve vehicles side-swiping each other, which is much less dangerous.
Roundabouts also are often better at letting traffic flow than stop-and-go intersections, although design details are important. They may not work well, for example, when the traffic flow in one direction is much greater than traffic flow in another.
Roundabouts are much more prevalent in Europe. Britain is a notable fan of the road design, to the point that they have many intersections with multiple intertwined roundabouts. The most famous in a town called Swindon involves five roundabouts built in a circle, which is known as the Magic Roundabout.
In general, roundabouts are smaller than old-fashioned rotaries. In the U.S., the main design points of a roundabout are:
∎ They always give the right-of-way to the car already in the circle, to reduce backups of traffic in the circle itself.
∎ They encourage lower speeds, usually 25 mph or less, through design changes such as smaller radius, large angle of entry, and “splitter islands” that separate incoming and outgoing traffic.
∎ They have a relatively large central island that includes a low apron which tractor-trailers can drive over without damage, if that is needed to handle the tight curve of a small circle.