Minnesota Roundabouts: Improving Traffic Safety
For those interested in roundabouts, and particularly how roundabouts can improve safety, the Minnesota Department of Transportation (MnDOT) recently released a report that deserves attention. Spoiler alert – roundabouts are safe!
MnDOT continues to build more roundabouts on their state roads, believing modern roundabouts compare favorably in safety and operational performance to conventional or traditional intersections (stop sign or signal controlled). For new intersections, the initial construction costs are comparable. Life-cycle costs are also favorable to roundabouts than a traffic signal without sacrificing capacity. For these reasons, MnDOT has elevated modern roundabouts to at least equal status with other traffic control methods and routinely completes side-by-side comparisons to determine which is preferred for a specific intersection.
While these beliefs are all based on solid data, the data had reflected the experience of other states or limited intersections in Minnesota. By examining 144 individual sites, this study is the largest comprehensive review of roundabout traffic safety. The methodology included reviewing daily traffic volumes (MnDOT has records back to 1992) and crash data within 300 feet of the roundabout (reported crashes from 2006 through 2015 are available). MnDOT then calculated crash rates (number of crashes per one million entering vehicles) to normalize crash data across various type of intersection with different control and volumes and provide an equal basis for comparison.
And what did they find?
As we stated up top, modern roundabouts do improve safety with a headline of over an 80 percent decrease in fatal and serious injury crashes. Beyond the front page number, the report includes details split into three different types of roundabouts: single-lane, multi-lane (at least one leg with one circulating lane and at least one leg with two circulating lanes), and dual-lane (two circulating lanes all around). You can find interesting nuggets in this data as well, including:
– Single-lane roundabout operations showed a decrease in overall crashes while multi- and dual-lane roundabout operations had in increase in overall crashes.
– Single-lane roundabout operations had a decrease in multi-vehicle crashes while these crashes increased at multi- and dual-lane roundabout locations.
– Fatal crashes were virtually eliminated at all types of roundabouts.
– Rear end, left turn, and right-angle type of crashes had significant reduction.
– Sideswipe same direction was the largest increase in crash rate and crashes for a dual-lane roundabout.
In short, roundabouts are saving lives and improving the transportation system safety, although not without some challenges (looking at you dual lane roundabouts). This extensive study just proved it again.
Want to read the report? MnDOT has made it available to the public here.