Around-a-Circle

 A recent article in the New York Times highlights the city of Carmel, Indiana and what they are doing to improve traffic safety and reduce carbon emissions. Carmel has 140 roundabouts, the most in the United States, and plans to add over a dozen more. 

The Federal Highway Administration found that roundabouts improve safety to the tune of a 90% reduction in fatalities, 76% reduction in injuries, and a 35% reduction in all crashes. Roundabouts reduce congestion by reducing stops as traffic flows through the circle. Reduced stops also have environmental benefits, as hard accelerations increase emissions and reduce idling time. 

The Oregon Department of Transportation cites similar studies highlighting the benefits of roundabouts. The following graphic shows how roundabouts result in fewer collisions. They are fewer "conflict points."

A study in Mississippi found that roundabouts decrease carbon dioxide emissions by 56%. Despite the benefits, many are still hesitant to adopt the roundabout. Carmel's city engineer says "you can spit out fact-based data, but at the end of the day most of the general population is scared of things that are new and different... They are used to being told what to do at every turn."

According to ODOT, Oregon has 50 roundabouts with 2 in Portland (2010). If you pictured the intersection of SE Glisan and Cesar Chavez, like me, we're wrong. That intersection isn't a roundabout - it's a traffic circle. Washington County, MN provides the distinctions between the two. 

A traffic circle is "a series of T intersections." Each intersection has a stop sign for vehicles entering the circle, as opposed to yield signs present in a roundabout. Exiting the circle requires a turn instead of a "straight ahead movement" found in roundabouts. The comparison goes on to say "traffic circles have a low capacity and are inefficient; they are used primarily for visual appeal."

The synonymous nature of the two intersections has made it difficult to research them separately, as searches for either would return similar results. Many references to "traffic circles" or "roundabouts" claim they are different names for the same thing, but they are starkly different. 

A roundabout is beneficial due to its continued traffic flow in response to yield signs, not stop signs. The environmental improvements of roundabouts, reduced idling and hard acceleration, are missing from traffic circles. Roundabouts are designed to slow speeds for traffic within the circle and are often smaller in diameter than traffic circles. The high-speed merging happening in a traffic circle nullifies the implied safety benefits. 

I am curious as to why we would choose a traffic circle instead of a roundabout, especially when the City has put an emphasis on safety and the environment. Is it possible to turn a traffic circle into a roundabout? Is it simply about the removal of a traffic signal? In the words of academics who came before me "more research is needed."