T2.2 Congestion mitigation
Roundabouts
According to the Project for Public Spaces
Carmel [Indiana] has pursued an aggressive roundabout program since the late 1990s. So far the city has replaced 93 signal-controlled intersections with modern roundabouts, giving it more roundabouts than any other city in the US. The roundabouts have been installed at all types of intersections, from low volume residential streets to busy highway interchanges. Not only has this improved safety for all road users and reduced delays and noise and air pollution, but the city estimates that it saves Carmel’s drivers 2.5 million gallons of gas per year!
The Insurance Institute for Highway Safety (IIHS) has an overview of the research on roundabouts.
It also says:
Some states, such as New York and Virginia, have adopted “roundabout first” policies requiring that roundabouts be considered a preferred alternative when building new intersections or upgrading older ones if feasible (New York State Department of Transportation, 2011; Virginia State Department of Transportation, 2009).
(I wrote to Representative Doglio to suggest this as a possible policy for the state and she wrote back to say she’d been told it already was, but I haven’t been able to locate any statement of that.)
According to the Washington Department of Transportation, they reduce emissions, mostly by eliminating stopping and idling at intersections; and the costs over time of a roundabout versus a signaled intersection are comparable.
According to a news report on a recent study from the IIHS, they reduce accidents, especially serious accidents
A new study from the insurance industry-funded safety group found roundabouts not only helped reduce crashes in areas that would typically employ a traditional intersection but also they nearly eliminate the most severe kinds of crashes. These are right-angle, left-turn, and head-on collisions. Since drivers need to yield and slow significantly to maneuver through a roundabout, they are inherently safer and less prone to extreme crashes.
The study looked at Washington state, specifically, which has more than 300 roundabouts. The traffic circles were constructed between 2009-2015 and the safety group looked at crash data for the first full year after an area completed a new roundabout. Traffic volumes, unemployment, and miles driven were all factored in to produce meaningful results. Finally, the IIHS separated findings between two types of roundabouts: single-lane and two-lane. For this study, the IIHS looked at 98 single-lane and 29 two-lane roundabouts.
Each year, crashes at two-lane roundabouts dropped by 9 percent. At single-lane roundabouts, they decreased 7 percent per year. The results likely factor in that drivers become more used to roundabouts and how they flow with time. The IIHS said it’s not uncommon for crashes to initially spike. A 2012 study found crashes increased at two-lane roundabouts after they replaced traditional intersections. Overall, the Washington State Department of Transportation, which worked with the IIHS, found an overall 37 percent decrease in crashes.
The types of crashes that fell the most may be the most important takeaway. Fatal crashes dropped 90 percent, while crashes where a driver or passenger was injured sunk 75 percent. Pedestrian collisions also dropped 40 percent.
National Association of City Transportation Officials on miniroundabouts