The Institute examined 24 intersections in eight states that were converted from stop signs or traffic signals between 1992 and 1997. The results that emerged are remarkably favorable:

• Vehicular accidents overall declined 39 percent.
• Accidents resulting in injuries fell 76 percent.
• Accidents resulting in death or incapacitating injury plunged 90 percent.

“Roundabout construction should be strongly promoted as an effective safety treatment for intersections,” said the four-person team of Bhagwant N. Persaud, Richard A. Retting, Per E. Garder, and Dominique Lord.

For years some New Urbanists have promoted roundabouts, or traffic circles, as an alternative to standard intersections controlled by stop signs or traffic signals. Now the Insurance Institute has weighed in with a highly encouraging, mathematically sophisticated before-and-after study of “modern roundabouts” in urban, suburban, and rural settings in California, Colorado, Florida, Kansas, Maine, Maryland, South Carolina, and Vermont.

A modern roundabout, according to the Institute, is a circle “designed for very low traffic speeds, about 15 mph.” Entrances and exits are curved so that motorists must travel slowly — far different from the rotaries of decades ago, which typically allowed drivers to enter at 35 mph or faster. The Institute says a modern roundabout typically needs to be about 100 feet across so that it can be properly designed to slow the entering traffic.

Other safety factors
Though reduced speeds help explain roundabouts’ safety, other factors in their success are the elimination of left turns against oncoming traffic, the elimination of right-angle collisions, and a reduction in rear-end collisions. Raised “splitter” islands divide the roadway at the entrances and exits, providing refuge for pedestrians and at the same time separating opposing traffic. The Institute says roundabouts may help reduce traffic delays, vehicle emissions, fuel consumption, and noise. They can save local governments money by avoiding the need to buy, install, and maintain traffic signals.

		Roundabout designed by Michael Wallwork. A Gainesville, Florida, intersection shows typical design of a modern roundabout.

After a long period of being out of favor among traffic engineers, roundabouts have been built in growing numbers in recent years. “We use roundabouts often in both infill and greenfield design,” says Peter Swift of Swift and Associates, town planners and civil engineers in Longmont, Colorado. Roundabouts, Swift says, can accommodate high road capacity, give retail and commercial enterprises valuable exposure to traffic, provide pedestrian comfort, and create “a very safe multi-modal environment,” all at the same time.

That combination makes roundabouts “perhaps the most elegant solution to handling intersections and encouraging pedestrian activity,” Swift says. In many cases, roundabouts make it possible to slim a four- or five-lane signalized intersection into just two lanes plus a median or center turn lane, he notes. With fewer lanes, pedestrians have an easier time crossing the road.

Some designers express reservations. Mike Watkins, head of the Kentlands project in Gaithersburg, Maryland, for DPZ Architects, says a roundabout at the end of Main Street in Kentlands is a problem for pedestrians, especially the elderly, attempting to cross the busy Kentlands Boulevard.

Watkins says one of the virtues of a roundabout — its ability to keep traffic moving — is at odds with the needs of pedestrians who want to cross. In his view, a further disadvantage of roundabouts is that, “the crosswalk is pushed away from the intersection,” creating travel paths that are inconvenient for pedestrians.

Swift says the problem at Kentlands, where the roundabout in question was designed by a local engineer, is an example of an entirely different phenomenon — roundabouts that have been “designed wrong.” The Kentlands roundabout “was designed for relatively high-speed throughput,” Swift says. “There are wrong ways and right ways to design these things.”

In a correctly designed roundabout, pedestrians cross about 20 feet before vehicles enter the circle, Swift observes. Splitter islands, a critical element in slowing the traffic, can be designed with great esthetic interest, as can the roundabout’s center, he points out.

Transportation engineer Walter M. Kulash of Glatting Jackson Kercher Anglin Lopez Rinehart community planners in Orlando says properly designed roundabouts are safer for pedestrians because those on foot need to cross only a single direction of traffic at a time and do not have to be concerned about vehicles turning from unexpected directions. “A roundabout is a less direct route in terms of distance,” Kulash concedes, but the distance is usually more than offset by the fact that pedestrians don’t have to wait for a “walk” signal.

The authors of the Institute study did not have enough data to estimate the effects on pedestrian safety. The Institute said roundabouts are not appropriate for high-volume urban intersections. However, Swift points out that Michael Wallwork, a prolific Florida-based designer of roundabouts, designed a two-lane roundabout in a commercial, mixed-use location in Towson, Maryland, that handles 30,000 to 40,000 vehicles a day safely.

The full report, “Crash Reductions Following Installation of Roundabouts in the United States,” can be found at the Institute’s web site at http://www.highwaysafety.org.