Newswise — When traffic engineers plan the roads that eventually will accommodate traffic in new developments like this, the plans usually involve intersections with stop signs or signal lights. But the barren site of a future intersection might be an opportunity to consider another option for traffic management, the modern roundabout. These have been built by the tens of thousands worldwide. The main benefits have been to improve traffic flow and reduce injury crashes by as much as 75 percent compared with intersections controlled by stop lights or signs (see Status Report, May 13, 2000; on the web at http://www.iihs.org). But only about 1,000 roundabouts have been built in the United States.

"Transportation engineers, like everybody else, generally go with what they're used to, and what they're used to on U.S. roads is constructing standard four-way intersections equipped with stop signs or signal lights. Doing this means missing the benefits of roundabouts, so we'd like to encourage officials to consider roundabouts earlier and more often in the roadway planning process," says Richard Retting, the Institute's senior transportation engineer and author of two new studies that suggest how to overcome traditional impediments to building roundabouts.

One impediment is logistical: It can be costly and disruptive to tear up an existing intersection and replace it with a roundabout. The easiest way around this is to construct the roundabout to begin with, before an intersection with a traffic light or stop sign is installed. Another roundabout opportunity is when an intersection with a signal light is scheduled for major modification.

Institute researchers studied 10 intersections where roundabouts could have been constructed but weren't. Instead local officials either outfitted the new intersections with traffic signals or retained the signal lights at intersections that were undergoing major modifications. The researchers measured traffic volumes, monitored the number of crashes that occurred, and estimated vehicle delays and fuel consumption at the intersections with the signals. Results were compared with estimates of what could have been expected with roundabouts instead.

A key finding is that vehicle delays at the 10 intersections would have been reduced by 62-74 percent, saving 325,000 hours of motorists' time annually. Fuel consumption would have gone down by about 235,000 gallons per year, and there would have been commensurate reductions in vehicle emissions.

The safety benefits also are considerable. Previous research indicates that roundabouts reduce crashes by 37 percent overall — injury crashes by 75 percent — compared with intersections that have signals. Applying these risk reductions to 5 of the 10 intersections for which crash data were available, researchers estimated there would have been 62 fewer crashes over 5 years. There would have been 41 fewer injury crashes.

"If only 10 percent of the 250,000 intersections with signals in the United States were modified as roundabouts, the national safety and fuel saving benefits would be enormous," Retting points out, "and you can reap these benefits without as many logistical challenges if you 'think roundabout' from the very beginning of a roadway project, for example when new housing or shopping developments create the need for roadway construction. Then it can be less expensive to construct a roundabout than to install traffic lights. Plus the developers may be required to fund the roundabout construction as a condition of zoning approval.

Initial opinion may be an impediment: Study after study, including the Institute's most recent one in northern Virginia, indicates the benefits of roundabouts in reducing both crashes and traffic congestion. Yet roundabouts frequently run into opposition, especially before they're constructed.

Institute researchers conducted telephone surveys of residents in three communities in New Hampshire, New York, and Washington State where intersections with stop signs or traffic lights were being replaced with roundabouts in 2004. The opinion surveys were conducted before the roundabouts were built and twice more, about six weeks after construction and then about a year later.

Fifty-four percent of the survey participants initially said they opposed roundabouts. One-third said they were strongly opposed. These proportions changed considerably right after construction, as motorists began getting used to the roundabouts. Then only 36 percent said they were opposed, and the proportion in favor increased from 36 to 50 percent.

"It might not sound like much of a victory to find out that half of the respondents expressed their approval for roundabouts. But the first follow-up surveys were conducted soon after motorists began navigating this new form of traffic control. Roundabouts weren't yet routine," Retting explains. Opinion surveys conducted more recently show growing approval. More respondents now say they like the roundabouts, while fewer say they disapprove.

Previous before-and-after surveys have revealed similar turnarounds in public opinion (see Status Report, July 28, 2001; on the web at www.iihs.org). This is because many motorists find out, through their own experience, that vehicles generally flow more smoothly through roundabouts than through intersections controlled by traffic signals. Delays are reduced. In many cases there's no need to stop at a roundabout, just slow down.

Message for transportation officials: "What these two studies teach us is simple. Just build them. Go ahead and construct a roundabout where it's appropriate, and do it, if possible, when a roadway is first engineered," Retting advises. Especially in suburban areas where population growth and housing development are escalating and new roads are planned, officials would do well to consider roundabouts.

"Don't let initial opposition get in the way," Retting adds. "Many U.S. motorists aren't familiar with roundabouts yet, so they're wary of them. But once the roundabouts are built, the traffic flow and safety benefits turn people around, even people who weren't enthusiastic from the get-go."

Publication quality photos and broadcast quality video available