What is a roundabout?

I have noticed a lot of confusion in Sacramento amongst both transportation professionals and citizens interested in traffic calming about roundabouts. If you live, work or play in downtown and midtown Sacramento, you’ve seen a lot of structures in the intersections which people call roundabouts, but which are not, they are traffic circles. So what is a roundabout?

intersection conflict points
intersection conflict points

A modern roundabout is a structure that allows a free flow of traffic without stop signs. Instead, they use yield signs and markings. They are most appropriately used at street intersections where both streets are fairly busy. The biggest advantage of a roundabout, for all users, is the reduction of conflict points in the intersection from thirty-two to four, as illustrated in the diagram at right!

modern roundabout (ContextSensitiveSolutions.org)
modern roundabout (ContextSensitiveSolutions.org)

Roundabouts are not traffic circles, which are small, or rotaries, which are large. The following text from FHWA is instructive:

A modern roundabout has the following distinguishing characteristics and design features:

  • Channelized approaches;
  • Yield control on all entries;
  • Counterclockwise circulation of all vehicles around the central island; and
  • Appropriate geometric curvature to encourage slow travel speeds through the intersection.

Modern roundabouts are different from other types of circular intersections in use in some parts of the United States. Roundabouts are typically smaller than the large, high-speed rotaries still in use in some parts of the country, and they are typically larger than most neighborhood traffic calming circles. Further discussion can be found in the Roundabout Guide.

Roundabouts are great solutions where streets are being built from scratch, significant reconstruction is taking place, or road widths are being narrowed. They are not usually good solutions for existing intersections in urban areas because they take up more space, space which must be gained by using some of the corner properties. If the existing streets are 42 feet or less, which is typical in traditional urban street design, there is just not enough room for a roundabout. However, when wider intersecting streets are being narrowed from three or four travel lanes to two, enough space may be gained from the travel lanes and parking lanes to create a roundabout.

There are concerns about the use of roundabouts by blind pedestrians. Since vehicles usually do not come to a stop, the blind pedestrian may have a harder time identifying a gap in which to cross. Active research is taking place to develop solutions to the challenge, and several enhancements are already in place at recent roundabouts.

Bicyclists also express a lot of concern about roundabouts. In a properly designed roundabout, bike lanes end before the roundabout. Roundabouts where the bike lane continues are not properly designed, and are dangerous. In a modern roundabout, the bicyclist can choose to either “take the lane” through the roundabout, or to use parallel sidewalks or paths navigate around the outside of the roundabout. The best roundabouts, as shown the photo above, have ramps for the bicyclist to go up onto and later exit the sidewalk or path. Many bicyclists who have not used roundabouts and/or are not used to taking the lane are afraid of roundabouts and think they don’t work. However, having used a wide variety of roundabouts in many cities with widely varying traffic conditions, I can say that they are easy to use and safe.

Large vehicles need larger turning radii. Modern roundabout design handles larger vehicles by having an apron around the center island, and sometimes around the approach medians. Large vehicles can mount these aprons, but must slow down even more to handle the low curb and roughly textured surface, which is exactly the intent.

Multi-lane roundabouts are a beast of a different color. Though they are in many cases better than traditional intersections, they are harder for motor vehicle drivers, bicyclists and pedestrians to use, particularly blind pedestrians. They introduce the multi-lane threat, which is not present in single-lane roundabouts. Therefore, what I’ve said above and my strong support of roundabouts does not necessarily apply to multi-lane roundabouts. Though there does not seem to be an adopted terminology, I think these should be called complex roundabouts, because they are, and because they need to be distinguished in all cases from single-lane roundabouts.

In the Sacramento area, there are modern roundabouts in Davis, West Sacramento, North Natomas, Roseville and Folsom. There are also a number of mis-designed roundabouts. Unfortunately, I have few photos of the good and the bad. If you do, please pass them along and perhaps we can create a Flickr group of Sacramento area roundabouts.

traffic circle, 13th & F, Sacramento
traffic circle, 13th & F, Sacramento

Traffic circles have their uses. They can be placed in the existing footprint of an intersection, not requiring any additional right-of-way or major reconstruction. Traffic circles have some traffic calming effect, causing vehicles to move more slowly due to the deflection. But they have far less traffic calming effect than roundabouts. Some traffic circles have two stop signs, some have four stop signs, but the use of stop signs eliminates the free flow of traffic. Some vehicle drivers are confused by traffic circles and behave erratically in them, which can also be true of roundabouts, but with so much less benefit from a traffic circle, it may not be worth it. Traffic circles reduce the number of conflict points from thirty-two to twenty, still more than four for a roundabout. Traffic circles can beautify a neighborhood, if planted for low maintenance and high color, or with public art.


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