Roundabout study provides guidance for improving safety

Roundabouts are a fairly recent addition to the road system in the United States, and their relative newness has made them a topic of discussion and debate. While roundabouts dramatically reduce the incidence of fatal and severe-injury crashes compared to traditional signalized intersections, drivers continue to misunderstand the rules of the roundabout, resulting in improper use and avoidable collisions.

Extending solid-line striping from 50 to 250 feet before a roundabout helps drivers choose and remain in the correct lane. Extending solid-line striping from 50 to 250 feet before a roundabout helps drivers choose
and remain in the correct lane.

In a study funded by the Minnesota Local Road Research Board, researchers in the Minnesota Traffic Observatory (MTO) at the U of M examined driving behavior and safety before and after signing and striping changes were applied at a two-lane roundabout in Richfield, Minnesota. 

This roundabout, built in 2008, exhibited an abnormal number of crashes after its completion. In response, local engineers experimented with changes in the roundabout’s signs and striping.

Researchers led by MTO director John Hourdos analyzed crash records and examined hundreds of hours of video to compare the crash rates and number of violations committed by drivers before and after the changes.

The findings indicate that the changes in signing and striping have made the Richfield roundabout safer. In particular, extending the solid line leading up to the intersection approach from 50 feet to 250 feet seems to have reinforced the message to drivers that they must select the correct lane before approaching the roundabout entrance. This reduces the occurrence of drivers turning improperly and the need for a driver to change lanes within the roundabout. 

Another important finding was that the traditional fish-hook-style roundabout signs and complex striping patterns often cause confusion among drivers. “Getting rid of the fish-hook signs and simplifying the striping really made a difference,” says Richfield city engineer Kristin Asher. “Our biggest problem before the restripe was left turns from the outside lane causing conflicts and crashes. Once the fish-hook signs were replaced with traditional lane designation signs and the skips were removed from the circulatory lanes, those crashes essentially disappeared.”

Prior to the changes, left turns from the outer lane accounted for 45 percent of the recorded crashes. Immediately after the changes, the occurrence of improper turns decreased by 48 percent and incorrect lane choice was reduced by 53 percent. One year after the changes, the safety improvements were still significant: the occurrence of improper turns was still down 44 percent and incorrect lane choice was reduced 50 percent compared to the “before” scenario.

Researchers studied this roundabout in Richfield, Minnesota.

Researchers studied this roundabout in Richfield, Minnesota.

To improve safety and decrease driver confusion, Hourdos says it may be necessary to look beyond the current design guidelines for roundabout markings, which are still relatively immature. For example, there is no specific guideline in the Manual on Uniform Traffic Control Devices for the length of the solid line between lanes at roundabout entrances. This research indicates that extending the solid line improves safety by helping drivers select the correct lane.

“This is an area where improvements can be made to the current guidelines,” he says.

A research brief summarizing MTO roundabout research—Safety and Risk in Modern Urban Roundabouts—is on the CTS website.

Research persuades residents of roundabout safety

Because roundabouts are relatively new to the United States, engineers and project designers are often faced with the challenge of persuading a skeptical public at the start of new roundabout projects. That’s exactly what happened when a new roundabout was planned on a local thoroughfare in Woodbury, Minnesota. “There was a very engaged neighborhood group near the intersection, and they were concerned that children couldn’t cross the roundabout on foot and that it would cut off their access to local parks,” says Bill Klingbeil, an engineer with the project’s design firm HR Green. 

When project designers spoke with the opponents, they learned that very few of them had actually crossed a roundabout. To counter their concerns, Klingbeil shared the results of a bike and pedestrian roundabout study conducted by MTO researchers (read about the project in the November 2012 Catalyst). “In this study, more than 3,000 people safely crossed a roundabout even busier than the one we were planning with no close calls or accidents,” says Klingbeil. “The project opponents had a strong opinion on something they had never experienced, and this research helped persuade them that roundabouts are safe for pedestrians.”