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Status Report, Vol. 43, No. 4 | June 9, 2008 Subscribe

Roundabouts can be even safer with easy changes

Roundabouts are vastly safer than traditional intersections, and most drivers like the circular intersections once they get used to them. Still, some mostly minor collisions do occur at roundabouts, and some motorists find them confusing or worry they're unsafe.

One common type of crash happens when a vehicle runs off the road, likely because the driver didn't see the roundabout in time or slow down soon enough. Relatively simple changes like better lighting, pavement markings, and landscaping could reduce crashes by helping motorists navigate roundabouts more safely.

In the first formal analysis of crashes at U.S. roundabouts, Institute researchers examined police reports for crashes that occurred at 38 locations in Maryland. The state was an early adopter of the modern roundabout, introducing the first one in 1993 and building a total of about 46 by August 2005. The review includes 149 crashes at 29 single-lane roundabouts and 134 crashes at 9 double-lane roundabouts.

"Modern roundabouts virtually eliminate the most serious kinds of crashes that occur at traditional intersections controlled by traffic signals or signs. Because they keep traffic moving, they handle more vehicles at once than traditional intersections can, saving fuel and time," says Anne McCartt, Institute senior vice president for research and an author of the study. "When crashes occur at roundabouts, they tend to be minor, mostly involving just property damage. Relatively simple enhancements can make existing roundabouts even safer."

Where roundabouts have been installed, crashes have declined about 40 percent, and those involving injuries have been reduced about 80 percent (see "Roundabouts sharply reduce crashes, study finds," May 13, 2000). More than 1,000 roundabouts have been built in the United States, and many more are planned or under construction. Support for roundabouts has increased over time in communities where they've been installed, as people get used to the new traffic patterns. Motorists who disapprove of roundabouts most frequently cite safety concerns and confusion about how to navigate them (see "Drivers may think they won't like roundabouts, but they end up being fans," July 28, 2001, and "When roadway design options are wide open, why not build a roundabout?" Nov. 19, 2005).

U.S. roundabouts feature a raised center island that vehicles travel around in a counterclockwise pattern. Entering traffic yields the right of way to circulating vehicles. The center island and the tight radius of entrances and exits help to keep travel speeds down to about 15-20 mph in urban areas and about 30-35 mph on rural roads. Slower speeds help vehicles merge more easily and reduce the severity of the crashes that do occur.

Three main types of crashes account for a majority of those that occur at roundabouts, according to European and Australian studies: collisions between entering and circulating vehicles, run-off-the-road crashes, and rear-enders. In Maryland, Institute researchers identified sideswipes as a fourth type prevalent at double-lane roundabouts. About three-quarters of the crashes in the study involved only property damage, and no crashes were deadly.

Crashes happened disproportionately at entrances to roundabouts. About 80 percent of collisions at single-lane roundabouts and about 60 percent of those at double-lane roundabouts occurred at entrances. Fifteen percent of the crashes at single-lane roundabouts and 28 percent at double-lane roundabouts involved vehicles traveling in circulating lanes. The rest of the crashes (about 4 percent at single-lane roundabouts and 12 percent at double-lane roundabouts) occurred at exits. Running off the road accounted for half of the crashes that occurred at single-lane roundabouts and 28 percent at double-lane ones. A common crash pattern involved vehicles running into center islands. This is how almost half of the run-off-the-road crashes occurred. Other major crash types included rear-enders and collisions involving an entering and a circulating vehicle.

Pedestrians or bicyclists accounted for just 6 of 283 total crashes. All of these happened at 2-lane roundabouts.

Crash patterns varied by time of day. Run-off-road crashes accounted for more than 60 percent of the evening/nighttime crashes at both single- and double-lane roundabouts, compared with 35 percent of daytime crashes at single-lane roundabouts and 9 percent of daytime crashes at double-lane ones.

"Speeding was a big problem in many of these crashes, and some of the drivers might not have seen the roundabouts in time, especially at night," McCartt points out. "The challenge is getting drivers to recognize roundabouts and then slow down as they approach and enter them. Design changes like narrowing the entry lanes, adjusting the curvature of the approach roads, and lengthening the splitter islands that separate roundabouts' approach and exit lanes can help."

Newer roundabouts tend to be better designed than previous ones, taking into account lessons learned from the earlier ones. But localities with older roundabouts don't have to go back to the drawing board. The Institute's study suggests that small changes can further enhance the already-substantial safety of roundabouts.

The most effective changes might be inexpensive ones. Reflective pavement markers and large "roundabout ahead" and "yield" signs could help alert drivers to roundabouts and the need to slow down and yield to circulating traffic. More shrubs and brighter lighting could help drivers better spot center islands.

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