Improving safety of two-lane roundabouts
Adding roundabouts to intersections on busy roadways is becoming increasingly common in the United States, and while two-lane roundabouts almost always reduce fatal and severe crashes they can also lead to a substantial increase in minor crashes. To help combat this problem, researchers with the Roadway Safety Institute (RSI) are continuing to expand on previous research investigating solutions for reducing crashes at two-lane urban roundabouts.
The research, funded by the Minnesota Department of Transportation, began in 2011 with a troublesome two-lane roundabout in Richfield, Minnesota—at the time, one of the few modern urban roundabouts in the Twin Cities. Following the roundabout’s construction, the number of property-damage crashes increased substantially at the site. Experts at the Minnesota Traffic Observatory (MTO) were brought in to make improvements, then gauge their effectiveness, with a before-and-after study.
“With the assistance of local and national roundabout experts, we made a number of changes in the signs and lane markings at the roundabout,” says MTO director John Hourdos, who is leading the research. These changes included adding lane designation signs upstream of the approach, removing fishhook-style arrows, extending the solid lane line upstream of the roundabout, eliminating the solid-and-skip lines, lowering signs to improve visibility, and adding crosswalk signs on the islands.
The effects of these changes were documented with a 360-degree camera mounted in the center of the roundabout; video was collected and logged before the changes, three months after the changes, and again one year later. Based on these data, the researchers concluded that a number of the changes had a significant, positive long-term effect.
“After the changes, lane-change violations decreased 20 percent, turn violations dropped more than 40 percent, and lefts from the outer lane—the most severe violation—were reduced 45 percent,” Hourdos says. “One early clue that led us to believe that the changes were significant is that we immediately saw a 53 percent improvement in selecting the correct lane—which was sustained one year later.”
Following the success of this single project, researchers wanted to extend the investigation to more roundabouts with different designs. “We knew this would allow us to solidify our ideas about what can be done to help improve roundabout safety.”
In a new project, funded by the Minnesota Local Road Research Board, the research team is accomplishing this by studying four additional two-lane roundabouts in Minnesota that vary greatly in age, driving conditions, and built environment. Researchers have collected before-and-after data at all four roundabouts, and video analysis is currently under way, with results expected later this year. The project also includes the use of automated violation detection for the video analysis.
Hourdos says the team has adapted open-source code to automatically detect yield violations on the video collected. “Previously our students were watching the video and logging violations manually, so this change will allow us to analyze a lot more video and gather much more useful data.”
Based on the results of the data analysis, researchers will develop guidance for Minnesota county and municipal engineers seeking to improve safety and reduce property damage crashes at their urban two-lane roundabouts.
“We made a lot of changes to our two-by-two roundabout in Washington County based largely on the results from the earlier Richfield study, and those changes had a big positive effect on turning violations, but we believe there is still room for improvement in yield violations,” says Joe Gustafson, Washington’s County’s traffic engineer. “We’re looking forward to seeing the results from this latest study to determine what additional treatments might be effective in reducing our crashes even more.”
Once this latest project is complete, researchers hope results may pave the way for a larger, multi-state study.