[ghaff]

There's one roundabout between me and Boston that gets very congested at rush hour and I commuted into on and off for about 18 months. It was obvious when school started up because the backups got way worse--partially more cars I assume but presumably you also had a lot of people who just weren't used to the traffic patterns. It tended to get better (though still awful) again after a few weeks.

ADDED: (One) problem with Route 2 is that it's an arterial highway that was never designed to be one. Especially to the west, the merges are also terrible at peak times. And (although there was one major upgrade a number of years back) it passes through some of Boston's tonier suburbs which makes major changes hard.

[JoeAltmaier]

Roundabouts are extremely sensitive to rates of traffic. If one direction occasionally fills from e.g. a school parking lot, the others can become instantly congested.

You can use traffic lights etc to fix this. But then the major benefit of roundabouts is extinguished: their cheap cost. Just some pavement and acres, painted lines.

[ryandrake]

They're great for low traffic neighborhoods where the intersections don't warrant something as heavy weight as stoplights or 4-way stops. I'd love to replace every ridiculous 4-way stop in my neighborhood with a roundabout, it would probably double the life of my brake pads and have no effect on safety. Except Americans are baffled by them and they'd probably get shouted down at the next city council meeting for being weird and confusing.

[SoftTalker]

I have thought that a "zipper merge" (let one car in ahead of you) convention would help congested roundabouts but that isn't how we're taught to use them.

[ghaff]

I'm not sure how that would work. Now the person entering the rotary has to trust that another driver isn't going to cut them off. I can tell you that merges right after a traffic light it is absolutely routine for a car (or probably more likely a pickup) not to let you alternate. I don't even bother with the FU horn any longer.

[AndrewStephens]

When I moved to Boston I was supersized that people would talk in fear of all the newfangled "rotaries" as they are called here. Roundabouts in general are awesome but Boston has some unique challenges.

Roundabouts work well when there is similar amounts of traffic coming from all directions. In Boston they built some roundabouts where arterial roads meet suburban streets (or country lanes) at odd angles and it causes congestion. There really isn't a great solution for those intersections though - lights would reduce traffic flow.

[ghaff]

It's basically the Route 2-related rotaries. The Concord rotary and the "twin doughnuts of death" as you enter Cambridge. Of course, all those intersections would be nightmares at rush hour however they were designed/configured.