Traffic Circle

Is a TrafficCircle what we brits would call a roundabout? I was going to say that it sounded silly, but in retrospect, roundabout sounds kind of silly, too. -- DavidMcNicol

Yes. In Massachusetts, they call them Rotaries and they can be a nightmare during rush hour.

Rotaries are famous in Massachusetts! It's like every one going into battle from 5 directions and playing chicken with each other at some of them (Concord Mass.) -- sg

There are a couple of interesting articles on Wikipedia about this, which tend to suggest that there has sometimes been a distinction between traffic circles and roundabouts. It also reveals that the earliest traffic circles built in the US had terrible congestion problems because priority was given to traffic entering the circle. Now, we programmers know how difficult it can be to correctly manage shared access to a resource, but to make a mistake like that is stunning.

I live in the city of Perth, UK, where we have millions of mini roundabouts to humongous ones with part time traffic lights at rush hour. Everybody in our city understands them.

An argument that traffic circles make residential neighborhoods safer is given at:

The advantages (and disadvantages) of traffic circles are:

  1. They make people drive at 15 miles per hour (25 kmph) instead of 30 mph (50 kmph).
    • Disadvantage: This also slows down fire engines.
  2. You can only make right turns, thus avoiding left-turn accidents. (swap L and R in the UK)
  3. There aren't any yellow lights, so you don't have to choose between running a yellow light and getting rear-ended.
  4. They discourage driving along streets that have them, promoting less traffic, and more walking. :)
  5. It is a lot easier to make a U-turn.
  6. Cars merge naturally into gaps in rotary traffic, creating a more even traffic flow than is possible with a traffic light and supposedly causing less backup.
    • However, if there are two major roads entering the traffic circle, it can get so busy that no one on the minor roads will get a chance!

Traffic circles are also a misfeature in WashingtonDc & NewJersey. Some of them have traffic lights, which rather defeats the purpose.

Like a CropCircle, a TrafficCircle is a clear sign of extraterrestrial influence. We all know mere humans could never build something so stupid. -- PCP

What's stupid about them? You guys are just amateurs. In the UK we have a roundabout who's entries and exits are other smaller roundabouts tangential to it. On a small island like Great Britain it makes more sense to use roundabouts than those huge sprawling clover-leaf junctions used...elsewhere. We did build "Spaghetti Junction"

Now, that sort of thing is stupid. (click on the little camera icon to appreciate the full horror of the situation.) Doesn't look much worse than a standard cloverleaf interchange - of which we have thousands. OK, it's a little irregular, but the roads aren't coming in at right angles, and it looks like you have limited space. -- JeffGrigg (...who hails from LosAngeles, "land of freeways." ;-)

I am seeing instances of traffic circles - roundabouts - in Phoenix, Arizona. What's distressing is that they're showing up in places that make no sense - residential intersections with very little traffic. I think they're being used just to be cute, but people here are not used to them and don't understand the rules. Most people I see look rather confused encountering them. I have to think that the accident rate at these things is a little higher than it would be for a regular four-way stop.

If only some had been built on Grand Avenue, eh?

Here's a traffic circle in the suburbs of Saint Louis, Missouri:

To the left (West) of the circle is a high school, to the right (and pretty much the whole surrounding area) is residential. It's really quite silly. None of these are major streets. (Hanley, on the left, is good for through traffic, but the main East-West corridor streets are to the North and South (Olive and Delmar, if you zoom out far enough to see them).

Traffic circles are used in residential neighborhoods as a way of slowing traffic. Usually what happens is that a street will represent a straight shot from one side of a neighborhood to another, and will be driven at higher speeds than are prudent. Usually these are long straight streets that parallel major streets that are congested. -- RobertField

What is wrong with a simpler solution - a STOP sign? No confusion, less space -- PeteHardie

The problem is not with the observant driver who will drive the residential streets at 25 MPH. They will stop at the stop signs. It is the drivers who would go 40 MPH, ignoring stop signs. Only a physical barrier will slow them down. Example, from Mountain View, California - Latham/Church Street. About 15 years ago the access from Church to Highway 237 was closed off. That solve the problem of people zooming on/off the highway onto Church Street. But the residential street was still used for "high speed" access to/from Castro Street, and so traffic circles were put in about five years ago. Apologies for non-Silicon Valley types for the length of this post. The entire practice is called TrafficCalming. -- RobertField

The most economical solution is to stop fixing the road. Allow pot holes (chuck holes for some of you) to develop. They are nature's speed bumps. I've seen a city pave over perfectly good pot holes just so they could put speed bumps on top of them. -- EricHodges

They only work for the rare individual that actually obeys traffic controls. A physical barrier is generally more effective. I favor 2-foot high speed bumps. -- That's not a SpeedBump, it's a ramp! Fun.

I favor application of lead pellets at high velocity to the offending vehicle. Why inconvenience all to stop the idiots, when you can eliminate the idiots? -- PeteHardie

How strange... in the UK roundabouts are used to keep traffic moving fast and smoothly, rather than slowing it down.

My favorite wacky instance of TrafficCircle / roundabout has to be the MagicRoundabout.:

What's the point of the counterclockwise circle on the inside? No one should have to go in both directions around a roundabout -- that just increases the chance of collisions at each of the mini-roundabouts, as well as your chances of getting lost and missing your exit!

On a simple roundabout weight of traffic on a busy route can keep traffic from a less busy route joining the roundabout. The MagicRoundabout arrangement stops that happening.

The things Brits will do to avoid putting up stop lights :-)

We have plenty of roundabouts with stop lights! M40/A46 anyone?

That one is silly though. It's quite feasible to do 50mph or more round that, without the lights. There are also roundabouts with part-time lights, for added fun. Never seen what state the light up to when they first come on duty. Amber, I expect.

Here in suburban Maryland, we have roundabouts where there is no intersection -- i.e., in the middle of the block along a two-lane street. Really "roundabout" isn't the right term for these abominations, which could more accurately be described as "chicanes". I don't know about you, but "calm" isn't the emotion I feel when I come to road features that mirror those found on racetracks. -- SkyeXu

Ah, TrafficTormenting?

Bend, Oregon has started installing TrafficCircles? at major suburban intersections. One interesting feature is the RumbleStrip. The RumbleStrip is a ring around the landscaped core that is raised slightly and is rather rough. It took me a while to puzzle out the function of the RumbleStrip, but I think it is to allow oversized trucks (fire engines?) to be able to make it though the TrafficCircle, though at a very reduced rate. -- ScottElliott

Colchester, Essex, has a roundabout with traffic lights on the roundabout itself, so you can join it, get halfway round, and then get stopped by the lights. Pure madness!

Just to bring this slightly back OnTopic, let me mention that a TrafficCircle once led to some interesting math and computer science: PietHein (better known as the author of "Grooks" and the inventor of the Soma puzzle cube) invented a new geometric shape called the "SuperEllipse" to solve a design problem for a TrafficCircle in Stockholm. About a dozen years later, DonKnuth decided that it was more practical for Metafont to have a built-in capability to draw superellipses than to draw circles. So in Metafont, you draw a "circle" by approximating it with a very round superellipse. Metafont (WikiName: MetaFont) is an interesting program in its own right, in terms of mathematics, algorithms, and language design, but it is best known as the source of the Computer Modern fonts used with TexTheProgram.

In the United States, the RumbleStrips? referenced above are technically called "mountable curbs." They are used at the "apron" of a traffic circle and are intended to allow vehicles to pass over them at a reduced speed when necessary. They may have a ridged surface (to provide a visual indication that they are not part of the normal driving surface).

Strictly speaking, "rumble strips" are the lateral ridges cut into the normal driving surface (at the approach to a toll plaza, for example) or cut into the shoulder. The vehicle's tires make noise as they pass over the ridges and alert the driver to a potential hazard.

-- RJS

The thing I find amusing about visiting the UK is not only the plethora of roundabouts on the ground but in the sky more often than not coming in to London I find myself circling waiting for landing. A few times in other countries too but much less frequently than the UK which I've been to many times.

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