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Status Report, Vol. 36, No. 7 | July 28, 2001 Subscribe

Roundabout reduce traffic backups and crashes, too

Modern roundabouts reduce motor vehicle crashes — a big safety plus. Another important plus, compared with the stop signs that roundabouts often replace, is a reduction in dreaded traffic delays by eliminating the requirement for all motorists to stop. These are the probable reasons that even motorists who didn't like the idea of roundabouts to begin with end up liking them once they're installed and drivers navigate them a few times.

The finding that roundabouts reduce crashes, including injury crashes, isn't new. An Institute study found a 75 percent reduction in injury crashes at roundabouts converted from stop signs or signals (see "Roundabouts sharply reduce crashes, study finds," May 13, 2000). What's new is that roundabouts also reduce delays at U.S. intersections. This is a main finding of a recent Institute study conducted by researchers at Kansas State University, who measured traffic flow at three intersections in Kansas, Maryland, and Nevada before and after conversion to roundabouts.

"In each case, installing a roundabout reduced the amount of traffic having to stop at the intersection, leading to about a 20 percent reduction in delays," says Richard Retting, the Institute's senior transportation engineer. The proportion of vehicles having to stop declined 14 to 37 percent across all three sites. In turn, backups on the approach streets declined at all three sites, and delays per vehicle dropped 13 to 23 percent.

These improvements were found at intersections where roundabouts replaced stop signs — the main use of roundabouts so far in the United States. Other studies have found substantial traffic flow improvements at intersections where roundabouts replaced traffic signal lights.

Reduced stopping means fewer traffic delays

"Stopping is the number one cause of delay at intersections, so when you eliminate the requirement to stop, you reduce delays," Retting explains. "Because roundabouts keep most traffic moving, they can handle more traffic in the same amount of time compared with stop-controlled intersections. Traffic might only be moving at 15 mph, but even with the reduced speeds it's more efficient because you're not requiring every vehicle to come to a complete stop."

A roundabout also improves access to intersections for motorists approaching from minor roads. This helps to reduce delays that can occur when volume is a lot heavier on some approaches than others. Stop signs and signals are less efficient because they require at least some traffic to stop. They're also less safe.

"The most serious kinds of crashes at conventional intersections are virtually eliminated by roundabouts," Retting points out. "Crashes that do occur tend to be minor because traffic speeds are slower."

Experience changes opinions

Roundabouts still haven't been as fully accepted in the United States as elsewhere. For example, they're widespread in the United Kingdom, and engineers in France are building upwards of 1,500 new roundabouts per year. Public resistance in the United States is part of the problem, but the resistance is expected to fade as more communities build roundabouts. Public opinion surveys conducted in the three communities show more than a doubling of support for roundabouts after they're built.

Percent reductions in traffic delays and stopped vehicles after roundabouts

Graph image
Roundabouts prove popular

Seeing is believing for many people, and experiencing roundabouts can convert skeptics.

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