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Status Report, Vol. 35, No. 5 | May 13, 2000 Subscribe

Roundabouts sharply reduce crashes, study finds

Roundabouts may be an unfamiliar type of intersection in the United States, but they're becoming more familiar as evidence of their benefits grows. Improved traffic flow, aesthetics, and cost savings make roundabouts a good idea, and the safety gains are compelling. An Institute study shows far fewer crashes occur at intersections with roundabouts than at intersections with signals or stop signs. This is especially true of crashes resulting in occupant injuries. Researchers at Ryerson Polytechnic University, the Institute, and the University of Maine studied crashes and injuries at 24 intersections before and after construction of roundabouts. The study found a 39 percent overall decrease in crashes and a 76 percent decrease in injury-producing crashes. Collisions involving fatal or incapacitating injuries fell as much as 90 percent.

These findings are consistent with those from other countries where roundabouts have been used extensively for decades. They also are consistent with preliminary studies of U.S. roundabouts.

The safety benefits don't come at the expense of traffic flow. In fact, where roundabouts replace intersections with stop signs or traffic signals, delays in traffic can be reduced by as much as 75 percent.

"Given the magnitude of these crash reductions, there's no doubt that roundabouts are an important countermeasure for many intersection safety problems," says Institute president Brian O'Neill. "Replacing signals or stop signs with roundabouts will reduce the number of crashes and save lives while at the same time improving traffic flow."

The recent study focuses on urban and rural intersections in California, Colorado, Florida, Kansas, Maine, Maryland, South Carolina, and Vermont. The roundabouts replaced stop controls or traffic signals.

Old idea improved

Rotary intersections aren't new. They predate the automobile. In 1905, the first U.S. traffic circle, then known as a "gyratory," was constructed in New York City, and European countries built them in great numbers through the early part of this century.

In its basic form, a traffic circle consists of a raised island at the center of an ordinary right-angle intersection. The island, which directs cars counterclockwise, is intended to reduce speeds, although this goal isn't always achieved. Other configurations can be more complex. They may involve split lanes and combinations of yield signs, stop signs, and traffic lights — all of which can be confusing to drivers trying to negotiate them.

The modern roundabout improves on such designs. This is an important distinction, because the older traffic circles aren't always easy to navigate, so they haven't been very popular.

"At modern roundabouts, triangular islands at each entrance slow approaching vehicles," explains Richard Retting, Institute senior traffic engineer and an author of the study. "In older traffic circles, no physical structures prevent drivers from speeding right into the intersection. This lack of control contributes to high-speed conflicts inside the circle" — a problem solved by the islands at roundabouts.

Another feature is that vehicles approaching roundabouts yield to circulating traffic. No stopping is required. Some older traffic circles and many conventional intersections alternate traffic with stop signs or signals. Roundabouts enable all cars to move continuously through intersections at the same low speed.

"People assume that because there are so many traffic signals out there, they must be efficient. The fact is, they're not. When half of the cars are stopped at an intersection at any given time, delays are inevitable. It may seem counterintuitive that roundabouts increase capacity while lowering speeds, but that's exactly what happens," Retting points out.

Other design elements set roundabouts apart from traffic circles. Pedestrians cross only at the perimeter, vehicles can't turn left ahead of the central island, and parking isn't allowed inside the circle. These requirements minimize distractions and opportunities for collisions.

More common in other countries

The traffic calming properties of roundabouts may explain why they've been widely used in other countries but not in the United States. The American penchant for fast driving has created a culture where "slowing down" seems an encroachment on convenience. But the bias for speed isn't just among drivers. American universities and institutions that influence road planning and engineering have reinforced the historical practice of building high-speed intersections. Teachings haven't emphasized traffic calming as a preventive measure, at least not until recently.

"The priority for road planners and engineers in this country has been to process as much traffic as possible. Traffic signals have become the technology of choice. It's hard to deviate from that approach," O'Neill explains. "Countries in Europe and elsewhere have been much more progressive in focusing on traffic calming and making intersections safe for pedestrians. They caught on long ago to something we've ignored because of our fascination with technology. Recent interest in roundabouts in the United States is one sign that priorities finally are shifting."

Geometry eliminates the worst crashes

Roundabouts benefit from good geometry, exhibiting only a fraction of the troublesome crash patterns typical of right-angle intersections. Such intersections "place vehicles on a high-speed collision course, with crashes avoided only if drivers obey traffic laws and use good judgment. Research shows many drivers don't, so the potential is high for right-angle, left-turn, and rear-end conflicts," Retting explains. Such conflicts make up about two-thirds of police-reported crashes on urban arterials.

The geometry of roundabouts eliminates many of the angles and traffic flows that create opportunities for crashes, particularly the right-angle and rear-end kind that tend to produce injuries. The lack of right angles, combined with reductions in speed, make the intersections safer for pedestrians and bicyclists as well as people in cars. The speed depends on the intersection but generally remains at about 15 mph. At that speed, drivers and others on the road have more time to react, so there's a smaller chance of collision. When crashes do happen, most will be minor.

Fewer pedestrian crashes

Concern has been expressed that installing roundabouts might endanger pedestrians, but these fears appear unfounded. Experience in Europe shows roundabouts reduce the risk of pedestrian crashes. Such crashes also declined at the U.S. roundabouts in the new study, but the numbers were too small to be significant.

The combination of a rotary design and yield at entry, as opposed to right angles and stop controls, lends other safety benefits. Because there are no traffic signals to obey, drivers don't feel compelled to "beat the red light" or be first to cross the line when the light turns green. This not only reduces collisions but also takes the edge off at least one manifestation of aggressive driving. Plus the absence of a traffic signal and the curved roadway associated with roundabouts force drivers to pay attention to their surroundings, which further enhances road safety.

Cheaper, cleaner, and a nicer view

Roundabouts are becoming popular in the United States for more than just safety reasons. They're less expensive than intersections controlled by traffic signals, saving up to $5,000 per year per intersection in electricity and maintenance.

Fewer traffic snarls due to blocked intersections or backups can mean additional savings. For example, a pair of roundabouts introduced at a freeway interchange in Vail, Colorado, saves $85,000 each year in traffic control costs.

Environmental and aesthetic benefits add to the appeal. Roundabouts cut vehicle emissions and fuel consumption by reducing the time drivers sit idling at intersections. Traffic that moves more slowly through intersections creates less noise and congestion, minimizing the expressway look and feel of roads in urban and suburban areas. Landscaping on the islands replaces the asphalt of conventional intersections, which offers visual appeal and restores a bit of nature. Roundabouts also create visual gateways to communities or neighborhoods, and in commercial areas they can improve access to adjacent properties.

"Roundabout construction should be strongly promoted as an effective safety treatment at intersections," Retting concludes. "There's nothing to lose from constructing them and everything to gain. The proof is already there."

FHWA issues roundabout guide

The publication highlights best practices around the world and translates them in light of U.S. design standards.

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