When it comes to intersection design, I’m a big fan of roundabouts. If you follow me on Twitter you’ve probably come across more than a few tweets touting the benefits of roundabouts. From time to time I do radio interviews which are supposedly going to be about the Oilers, and even then I manage to squeeze in some talk about roundabouts. For an intersection requiring more traffic control than a two-way stop a roundabout is my intersection design of choice.
The reason is simple: they are often more efficient than a signalized intersection, and they’re safer. As long as we’re going to have intersection on our roads, and I don’t see them going away any time soon, safer and more efficient is probably a good way to go. Unfortunately roundabouts cost more to build than simply installing a traffic signal, and they require additional land, so while a roundabout is a nice option it’s not always a feasible option.
When I talk roundabouts with people I’m surprised at how often I hear the same two things. The first is that roundabouts are great, they’re fantastic in Europe, the problem is that nobody in Edmonton knows how to drive them, or really how to drive at all. Hearing that everyone thinks everyone else is the problem always makes me smile. The second thing I get a lot of, is roundabout and traffic circle being used interchangeably. This is often followed by a puzzled look when I explain that they’re not the same thing.
I can understand the confusion, they’re both circles and, in North America at least, you drive in a counterclockwise direction within the roundabout. That’s about the only real similarity though, and much of what make a roundabout a safer alternative to a traffic signal as I mentioned previously has to do with the differences between a roundabout and a traffic circle.
First you’ve got the approach. On the approach to a roundabout drivers are pointed at, or to the left of, the centre island and then deflected at the approach. This serves to slow the driver down, first with a fixed object in the driver’s line of sight and then with the sharp angle at the entrance to the roundabout. In the case of traffic circles, drivers are aimed to the right of centre allowing for a much higher speed entry into the roundabout. Most collisions are going to occur at the entrances, higher speed with result in more serious collisions.
Higher entrance speeds also mean higher circulating speeds, which in turn mean larger radius curves, and larger radius curves take up more space. So with the reduced entrance speeds at roundabout comes a reduced overall footprint when compared to a traffic circle. The picture on the right is of a large traffic circle in New York being converted to a roundabout, the difference in the size is immediately obvious, but this picture also shows the entries quite well. Which one do you think you could driver through faster?
Entering and exiting a roundabout if different as well. Drivers entering a roundabout yield to those drivers already in the roundabout. At a traffic circle drivers are not required to yield. In the reverse this means that drivers have a free flow exit from a roundabout, whereas at a traffic circle you have to find a gap to exit, like in National Lampoon’s European Vacation. A free flow exit gives roundabouts higher capacity than you would typically get with a traffic circle where entering traffic can bring the whole system to a standstill.
As you can see they really aren’t the same. And as for everyone else being able to drive them, that’s just going to take time.