We have a few 6 lane (and a bunch of 4 lane) roundabouts and rotaries here in Metro Detroit with more on the way. There is one area where two or three communities have a whole bunch of the roundabouts in close proximity to each other on some heavily trafficked roads and when they first started putting them in, everybody found something to complain about but eventually people got used to it. I cannot remember if the number of accidents actually decreased but I know that the severity of accidents is significantly less because everybody is (hopefully) moving in the same direction and at continuously similar speeds so there should be no head-n or t-bone accidents.
The one problem that we did run into after they were put in is that the engineers don't want to put in timed crosswalk signals because that stops traffic flow, which goes against the entire concept. So for pedestrians, not only does traffic not stop, but because of the design they often have double the number of lanes of traffic to cross. IIRC, the communities did finally convince the road commission to add HAWK Beacons to some of the crosswalks that have heavier pedestrian traffic.
We also have a diverging diamond interchange where an interstate, a major highway, and a major arterial street all come together. That one was pretty crazy but it really does help the traffic move.
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