MAINE home

Research Grants

News Releases Public Affairs

Calendar of Events

Department Directory

UMaine Today Research Archives

UMaine Today Research by e-mail Send message: "subscribe UMaine Today Research (your name)" Leave subject line blank.

Off Campus Links

Maine Science and Technology Fdn. for Maine research

ScienceDaily

SciNews/MedNews

Eurekalert

Humboldt Field Research Institute

May 19, 1998

Roundabouts Provide Solution to Traffic Problems

A promising solution to traffic problems in the U.S. may literally be going around in circles. Modern roundabouts, not to be confused with older generation traffic circles, have been installed in at least nine states from California to Maine and are already reducing accident and injury rates.

Engineers who design and study the new roundabouts have determined that properly designed circular intersections also have less traffic delay.

“The key to safety,” says Per Garder, associate professor of civil and environmental engineering at the University of Maine, “is slowing traffic down as it enters the roundabout. At 15 miles per hour, even if accidents occur, the probability of serious injury is close to zero.”

Garder received his Ph.D. at the Lund Institute of Technology in Sweden in 1982 and came to UMaine in 1992. Roundabouts are more common in Europe than in the U.S., and Garder includes them in courses on transportation engineering and advanced roadway design. In 1997, he advised the Maine Department of Transportation on the design and construction of a roundabout which carries an average 13,000 vehicles per day in Gorham, Maine.

Garder and his students have also developed designs for a new roundabout at the intersection of an off-ramp from I-95 and an arterial which carries 17,000 vehicles per day in Orono, Maine.

“Many people have faith in traffic signals,” Garder says. “However, signals are not fully safe, and if we can find a safer alternative, we should use it.”

He suggests that the critical design elements for the new roundabouts include the following:

• “Splitter islands” and “Yield” signs at roundabout entrances to slow and guide traffic and another splitter island at the exit.

• A tight enough radius in the circle to keep traffic moving at a speed of 25 miles per hour or less. A 50-foot radius keeps speeds to about 15 mph while a 175-foot radius is appropriate for 25 mph.

• A raised “apron” inside the circle to give large trucks space to make the turn. A rough surface raised at least two inches helps to keep cars from using the apron.

During its first year of operation, the new roundabout in Gorham has reduced the accident rate from 1.36 per million vehicles to 0.8. The previous intersection was controlled by stop signs which had rush hour delays of about sixty seconds for cars entering from the minor approaches.

Personal injury rates are also lower for the new roundabouts. On average, one in three accidents which occur at intersections with stop signs or signals have personal injuries. That compares to about one in ten in roundabouts. In Gorham, three accidents have occurred since the roundabout was constructed, and none involved injuries.

Most accidents in roundabouts involve cars which fail to yield to circulating traffic. As a result, collisions tend to be sideswipes rather than direct impacts at a right angle.

“There are three turning radii involved in designing roundabouts,” Garder explains. “First, the entering driver has to make a right hand turn. This is most important because the driver has to yield to circulating traffic. When he continues around the central island, he'll make another turn, and if we keep the radius short, very few drivers will attempt to accelerate. We need to keep a tight turning radius to keep speed low. Then, there is another turn at the exit. Again, that should be fairly tight.”

Older style traffic circles often fail to slow entering vehicles, says Garder. In addition, they may compound speed problems by having multiple marked lanes which contribute to unpredictable driver behavior.

Return UMaine Today Research home