Many of the well-documented benefits of roundabouts for intersection safety and efficiency extend to the two-lane variety, new Institute research shows. However, the study also sounds a note of caution about driver confusion when navigating these wider circles.
Researchers found that a pair of two-lane roundabouts built near Bellingham, Wash., reduced injury crashes and improved traffic flow. However, a year after construction, many people remained confused about how to navigate them, which may explain why researchers also observed an increase in noninjury crashes. Despite the confusion, public support grew quickly for the roundabouts after some initial wariness in the community.
"Roundabouts have many safety advantages over traffic lights and stop signs, and these projects in Washington resulted in real improvements," says Anne McCartt, the Institute's senior vice president for research and a co-author of the study. "At the same time, driver confusion is a potential pitfall. Two-lane roundabouts are inherently more complicated than single-lane roundabouts, so extra care is needed to ensure that the rules for navigating the roundabout are communicated."
Replacing a traffic signal or stop sign with a roundabout improves safety because the roundabout's tight circle forces drivers to slow down, and traffic flows in the same direction (see "When roadway design options are wide open, why not go ahead and build a roundabout?" Nov. 19, 2005). The most dangerous types of intersection crashes — right-angle, left-turn and head-on collisions — are essentially eliminated with roundabouts. And the low-speed rear-end crashes and side-swipes that sometimes do occur are unlikely to result in serious injury. Roundabouts also improve traffic flow and cut down on idling, which reduces fuel consumption and emissions.
Many of the roundabouts built in the U.S. in recent years have more than one lane. Because most research has focused on single-lane roundabouts, the Institute decided to document the effects of two-lane conversions.
The two roundabouts in the recent study are located about 5 miles north of Bellingham on Guide Meridian Road. One intersection, at Pole Road, previously had a traffic light, while the other, at Wiser Lake Road, had stop signs on the two minor approaches. The roundabouts were built as part of a broader reconstruction of the corridor, which also included widening the road from two lanes to four and erecting a cable barrier in the median.
Researchers looked at crash rates, traffic movement, fuel consumption and vehicle emissions at the two intersections before and after construction. Photographs of vehicles in the corridor were used to determine the age distribution of drivers. Researchers also conducted telephone surveys before the roundabouts were built, about six months after they opened and again after a year. More than 300 people were interviewed each time.
The rates of injury crashes fell after the roundabouts were built. At the Guide Meridian-Pole Road intersection, the rate went from 0.48 per 1 million vehicles to 0.21, while at the Guide Meridian-Wiser Lake Road intersection it fell from 0.28 to 0. Injury crash rates at comparison intersections, chosen for their similarity to the Guide Meridian intersections before the roundabout conversions, also fell, but not as steeply. Researchers estimated that the rate of injury crashes at the Pole Road intersection was 34 percent lower than would have been expected if it had not been changed to a roundabout, while injury crashes were eliminated at the Wiser Lake intersection.
The injury crash rate is arguably the most important measure and the one roundabouts would be expected to affect most. Although the decreases weren't statistically significant, possibly because the number of injury crashes at these intersections was small to begin with, they are consistent with benefits measured for less complicated single-lane roundabouts.
Also on the positive side, researchers found improvements in most measures of traffic flow and related reductions in fuel consumption and vehicle emissions.
On the other hand, the rates of crashes with only property damage rose after the roundabouts were built. Researchers concluded that noninjury crashes were 6 times as high at the Wiser Lake intersection as they would have been without the roundabout conversions. At the Pole Road intersection, they were 13 percent higher than they would have been.
"We don't know why noninjury crashes are so much higher than expected, but it may be related to confusion about right-of-way rules and other issues that drivers reported to us," McCartt says. "Nevertheless, these roundabouts are making travelers safer by reducing injury crashes."
The surveys revealed that even after a year many drivers continued to find the revamped intersections confusing. Nearly half of respondents said it wasn't clear from the signs and pavement markings which lane has the right of way when exiting or that they shouldn't drive next to large trucks in the roundabouts. More than a third said it wasn't clear what speed to drive.
Better signs might help. For example, one remedy the authors suggest for right-of-way confusion is for the yield signs at the roundabout entrances to make it clearer that entering drivers must yield to traffic in both lanes of the roundabout.
Observation data from cameras suggested some older drivers may avoid the roundabouts on Guide Meridian Road and instead take Hannegan Road, which runs parallel and can be used as an alternate route. After the roundabouts were built, drivers 70 and older were less likely to be traveling on Guide Meridian versus Hannegan Road than in the before period, although the number of older drivers was small on both roads.
Nevertheless, the roundabouts have gained popularity. As with roundabouts elsewhere, opposition faded as drivers became familiar with them. In this case, 53 percent opposed the roundabouts before they were built. That dropped to 44 percent six months after construction and 27 percent one year later.