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Roundabouts

Roundabouts as an intersection control method have been in use for many years in Europe, Australia and other parts of the world. Because of their superiority in certain situations, modern roundabouts (not be confused with traffic circles) became popular in the US as well. Right now there are over 2,300 of them in North America; with almost 300 new ones build every year. Some States even regulate that every new signal must be analyzed, to see if roundabouts are not a better option for traffic control.

Agencies in Michigan have recognized the benefits of roundabouts too. There are many of them in the State from the first in America mini roundabout in Dimondale to large three-lane roundabouts in Oakland County to the dreaded (but efficient) Lee Road twin roundabout on US-23 and Lee Road.  

The Michigan Department of Transportation have recently completed installations of roundabouts on some freeway interchanges (Maple Road and M-14 is one of many examples), and urban arteries. (Link to Maple Road and M-14 picture in Google Earth). This includes the Lee Road / US-23 interchange as well as M-53 and 26 Mile Road. The most recent freeway interchange converted from signals to roundabouts in our area is Geddes Road interchange on US-23. See plan of this project including City's Earhart roundabout and non-motorized path HERE.

The City of Ann Arbor built its first modern roundabout at the intersection of Huron Parkway and Nixon Road. This intersection was under All Way Stop control, but was failing from the capacity and safety points of view. In the process of upgrading the intersection, we performed a roundabout analysis, which showed that a small roundabout will reduce vehicular and pedestrian delays better than a signal might. In addition queues that were forming on Nixon in the morning and evening rush hours will be much shorter with the roundabout, but longer with the signal. The other issue resolved by a roundabout is the difficulty of coordinating a signal at this intersection with two Plymouth Road signals nearby. Finally, a roundabout may improve safety of this intersection, as lower speeds in general reduce the number but most importantly the severity of crashes. As a bonus there is much lower maintenance requirement compared to the signal, and fail safe operation under power outage, quite common during springs and summers in Ann Arbor. 

The Huron Parkway – Nixon Road roundabout was open to traffic in June 2009. Here’s a link to the Google Earth picture.

On October 22, 2010 the City of Ann Arbor together with MDOT opened Earhart / Geddes roundabout, which was designed for the City by MDOT and built together with MDOT's interchange of Geddes Road and US-23. The replacement of the existing signal at this intersection with the roundabout significantly improved operation of the whole system. Roundabouts work poorly with platooned traffic that is released by signals, and vice versa signals work poorly when they receive a random pattern of traffic. Therefore, systems containing signals and roundabouts are not compatible and that was one of the reasons the City decided to replace Earhart / Geddes signal with the small, single lane roundabout. Additional benefit of this project is a big improvement of non-motorized system by filling sidewalk gaps on Earhart and adding non-motorized path on Geddes with a separate bridge over US-23.

Washtenaw County Road Commission converted several of County’s intersections to roundabouts. Here is a link to the WCRC web site that contains a lot of information on roundabouts that might be of interest, both technical and popular. There is one IMPORTANT difference in the area describing how to navigate the modern roundabout. The new Ann Arbor law requires that vehicular traffic stops for pedestrians approaching the crosswalk. This is not covered in WCRC documents as they are not based on the City of Ann Arbor regulations. In Ann Arbor drivers need to stop for pedestrians in the crosswalk and yield to traffic in the circle. This is a very important difference to keep in mind.

 


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