As the name implies, mini-roundabouts are roundabouts that are smaller than the typical sized roundabout. The inscribed circle diameter of a typical roundabout is 132 feet. The mini-roundabout is supposed to have a diameter of less than 90 feet so it can fit inside a standard intersection.
The idea is that these mini-roundabouts should be cheap to build, provide the enhanced safety of typical roundabouts, and provide more capacity than a four way stop sign controlled intersection (similar capacity to a small intersection with traffic signal control). The Federal Highway Administration site and NATCO Urban Design Guide site are both good places to learn more about the concept.
The first mini-roundabout in Minnesota was built at County Road 79 and Vierling Drive in Shakopee, MN this past summer. The inscribed diameter circle is about 80 feet. Check out the Scott County site for more details on this specific project.
The intersection was all way stop sign controlled and had a crash rate of 0.9 crashes per million vehicle miles entering (nearly double the average in the metro area). My friend Rob teaches at the nearby school and the intersection backups were notorious. He says the backups are minimal now.
Mini-roundabouts are supposed to cost $100,000 to $200,000. This one came in at $338,000 (design and construction). That’s cheaper than the +/- $500,000 typically associated with a signal/turn lane project or the more than million dollar price tag of a full roundabout.
Jim McCarthy at the FHWA told me the mini-roundabout has been a big success with no crashes to date. We set up a COUNTcam last week at the intersection. Here are dropbox links: turning movement count data and video files.
We didn’t do a full delay/queuing study, but the backups are minimal and it appears to be operating at Level of Service A. A very successful project! Hopefully mini-roundabouts will become mainstream and the cost will come down.