Circular intersections really come in two flavors: One type is the “Big Ben/Parliament” type where you need to be on the outside to exit. This design is common in Spain, Germany, and the Netherlands. In this design, there’s a circle road, and entry and exit is a right-turn movement. You can get trapped in the inside lane, and you can circulate as many times as you like in the outside lane.

Most roundabouts in the US, however, especially those built in the last 20 years, are different. This design, which is the standard design in the US, Australia, New Zealand, and the UK, does not have a true circle road but only a circular island that separates one-way pairs. In this design, exiting the roundabout is a straight-ahead movement, not a right turn.

So, back to BSR’s comment: In the US/Aussie/UK design, you are NEVER supposed to change lanes within the roundabout. You ARE supposed to exit from the inside lane. Paths never overlap because the proper lane MUST be chosen before you enter the roundabout: keep left to turn left through the roundabout and exit directly from the left (inner) lane.
For cyclists, the information presented in this article is sage advice. Take the lane, stay centered, make yourself visible, and be alert for drivers doing dumb things. Use the pedestrian facilities if you like.
About the worst possible thing anyone could do as a cyclist is to try to ride around the perimeter of the roundabout and past an exit. Doing so in a US/UK/Aussie/NZ design equates to making left turns from the right-hand lane. It’s illegal and very dangerous. (BTW, it is also illegal for drivers to pass an exit in the outside lane).
A good visual is at http://goo.gl/maps/NGmGr. Notice how the left lane leads straight out of the roundabout, not around it.
Overall, roundabouts calm traffic speeds while reducing unnecessary waiting for drivers and cyclists alike. But user confusion remains a problem, as this thread indicates.

Missoula Montana has about 64 signalized intersections. The first 2 roundabouts on arterial roads (both will be single lane roundabouts) will be built in summer 2008. My group is thinking that a system of single lane roundabouts will work best. Yet, in order for the roundabouts to all be single lane, that means the approach legs can only be 2-lane roads, not 4-lane roads.

A 2-lane road (with either a landscaped center turn lane or periodic left turn pockets at minor intersections) can carry about 22k cars a day. An undivided 4-lane can carry about 26k cars a day. That is with signals. With roundabouts, the 2-lane can carry about 26k a day- the same as the 4-lane with signals! But the 4-lane with signals will have many more _severe_ crashes, stop and go traffic, noise and pollution, and higher land requirements. Pedestrians have been getting killed when one lane stops and a second inside lane does not.

In the context of a more sustainable transportation system, the 2-lane roads with single lane roundabouts can be coupled with great bus/train service, connected bike lanes and off-road trails, some car sharing and bike stations, and so on. Each city will be different, but I’m working on some of the principles to be put into motion- and soon.

Please email me at mist@strans.org if you have insights, comments, etc. thanks. -Bob Giordano, Missoula Institute for Sustainable Transportation.

That said, I do agree with everyone who pointed out that we Americans aren’t very good in roundabouts–either driving or designing them. Here in DC, they do all kinds of wacky things with them, including dropping in dozens of signals, which drives me bonkers, since they usually confuse the issue more than they do facilitate traffic flow.

Here in Germany, take the sidewalk, they have to stop for you, they tend to see you, and it is a safe and efficient way to get through the intersection.

http://www.youtube.com/watch?v=h3uFeauIRrA

If I remember correctly, roundabouts weren’t taught in drivers ed when I was a kid (it’s been a while) — does anyone know if it’s covered now?

Having driven quite a bit in Europe, my opinion is that most American roundabouts are poorly designed….most of them are too small.

Part of the problem seems to be the attempt to cram a roundabout into the space formerly occupied by an intersection (so they don’t have to deal with claiming more land, I guess). It’s not so bad, when it’s a single-lane — pretty easy for most motorists to figure it out.

Where I’ve seen many near-accidents is multi-lane roundabouts in a very tight space, with little or no signage BEFORE you get to it, so that people can plan their routes. If you’re trying to figure out where to go while you’re trying to figure out how this thing works, the result is often near-misses, sudden stops, or just complete vapor-lock while the hapless motorist’s brain tries to catch up. I’m now seeing these a lot in newly developed shopping areas and neighborhoods in the US. Some of them are double-lane for no clear reason — they don’t process enough traffic to warrant the additional complexity.

The roundabout in your picture appears to be one of the better-designed larger ones, but could still use better signage indicating which roads exit at each point. Some of the signs before entering roundabouts in Europe are HUGE to get all the names in, but they let you figure out where to go before you get into the thing. Then you can concentrate on watching other motorists.

As far as biking roundabouts — I’ve given up on some of them, resorting to routes that take me around, or (rather than wait for 10 minutes for a clear shot) just take the first exit, double back, and do it again until I’m on the road I need. Not optimal, but it feels safer that way sometimes. Where I live now (Denver) there are none on my regular routes, so I don’t have to deal with them on a regular basis.

Funniest moment I remember with roundabouts was about 20 years ago I was driving in the UK for the first time (left side) and was trying to drop a rental car off at Glasgow airport. We went through about 4 roundabouts in quick succession, but on the last one, we couldn’t find the right exit and ended circling one about 4 times. By the 3rd go-round, I was laughing so hard I could barely see! Fortunately there wasn’t much traffic right then!