Last fall the Economist had a short feature on traffic circles that I only noticed today. I was about to riff on how wonderful they are, and how underused in most of the US, but I’m already walking back the post I had in mind. For drivers, and people who breathe air, they are pretty much all win:

The Insurance Institute for Highway Safety, an independent research group, estimates that converting intersections with traffic lights to roundabouts reduces all crashes by 37% and crashes that involve an injury by 75%. At traffic lights the most common accidents are faster, right-angled collisions. These crashes are eliminated with roundabouts because vehicles travel more slowly and in the same direction. The most common accident is a sideswipe, generally no more than a cosmetic annoyance.

What locals like, though, is that it is on average far quicker to traverse a series of roundabouts than a similar number of stop lights. Indeed, one national study of ten intersections that could have been turned into roundabouts found that vehicle delays would have been reduced by 62-74% (nationally saving 325,000 hours of motorists’ time annually). Moreover, because fewer vehicles had to wait for traffic lights, 235,000 gallons of fuel could have been saved [MO: with attendant criteria pollution and noise reduction].

They have three defects, though, one tractable and two not so much. The easy one is that they reverse the convention of priorité à droite, the obligation of the car at the left of a merge to yield the right of way to the car on the right (this post assumes right-hand driving). This convention is not a problem when a minor street enters an arterial or avenue at right angles from the right (especially if it has a stop sign or a light), but the whole idea of a circle is that the circle itself and its tributaries have equivalent status and acute-angle convergence. In the new traffic circle, very rare in California, just set up in Berkeley, a few months of use and some yield signs seem to have done the job and we are zipping around and through it, correctly yielding to the traffic already in the circle.

Good as they are for vehicle traffic, however, they are really bad for pedestrians. Notice the Economist’s careless implicit definition of “locals” as “people who drive in the neighborhood”. Walking around a traffic circle of any size is either a long detour, crossing tributaries whose whole spirit is that their flow never stops, or navigating a one-way whirl, twice, that never has a red light and may be two or more lanes wide. When tributaries are light-controlled, things are almost worse: how do you get across DuPont Circle on your feet?…and that one even has a tunnel under it to divert some of the vehicle traffic. The aerial view in the Economist article rewards close attention: the sidewalks in this suburban wonder peter out in confusion and gravel and actually disappear as they get near it. Obviously the traffic engineer had no idea what to do next, and with good reason.

Circles larger than the little three-meter traffic calmers that substitute for (or, wrongly, complicate) four-way stops at minor intersections take up a lot of space, aggravating the effect of roads in driving origins and destinations further and further apart and causing sprawl. This space can be landscaped, but not used: the larger the circle the more unrelenting and unforgiving the traffic going around it that separates it from pedestrian life.

Too bad: another really seductive idea with fatal loose ends and crippling baggage.

[Cross-posted at The Reality-Based Community]

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Michael O'Hare is a Professor of Public Policy at the University of California, Berkeley.