It is common for people to use the terms roundabout and traffic circle interchangeably, as though there is no difference between the two. Sadly, Streetsblog San Francisco, whose mission is to educate the public, claims that there is no significant difference and that it is OK to use the terms interchangeably (SFMTA Launches “Muni Backwards” Program). That is not true.

A roundabout has two very significant features:

1. significant horizontal deflection which slows traffic
2. yield signs at all approaches

Traffic circles do not usually have these features.

The photo at right shows a roundabout in the River District/Township 9 in Sacramento. It is quite a bit bigger than a roundabout needs to be, and was installed in a new development, not at an existing intersection, but you can see the deflection and yield signs.

Significant horizontal deflection means that vehicles must change significantly from a straight line of travel, which requires that they slow significantly. This slowing reduces the number of crashes somewhat, and almost eliminates the number of severe crashes and fatalities (about 80% reduction). They are safer because they greatly reduce the number of conflict points in an intersection.

Yield signs on all approaches means that vehicles only have to stop for other vehicles already in the roundabout. Otherwise, they proceed at their reduced speed and never have to stop. This yield approach benefits motor vehicles and bicyclists who ride in the travel lane. It does not, and is not really intended to, benefit bicyclists in bike lanes or pedestrians, but at the same time, if does not hurt them. There is one standard MUTCD sign used at roundabouts, shown at right, but you will also see many other signs at both roundabouts and traffic circles.

Traffic circles vary widely in size, and therefore the amount of deflection. At least in California, they almost always have stop signs on two of the approaches, so that one street does not stop and the other does. The photo at right shows a traffic circle in downtown Sacramento. It has some deflection, but not enough to really slow traffic, and with the stop sign, does not ease the flow of traffic. These traffic circles also squeeze bicyclists who do not know they need to take the lane to safety navigate the intersection. The traffic circle sign is not a federal or state standard sign, but does communicate.

There are a a number of traffic circles in Sacramento central city, most of which were put in years ago, but a few newer ones also exist. Depending on the size of the circle, they have varying traffic calming benefits. The reason traffic circles are used instead of roundabouts is that you can’t just plop a roundabout into the footprint of an existing urban intersection. They require more space.

There are a lot more features of roundabouts than just the two that I mentioned. If you want to become a traffic nerd on roundabouts, I recommend the FHWA roundabout page, particularly the publication Roundabouts: An Informational Guide, Second Edition. For an international best practices perspective that focuses on bicyclist benefits, check Explaining the Dutch roundabout abroad.

Multi-lane roundabouts probably do not have significant safety outcomes over regular intersections. Many Sacramento people have experienced multi-lane roundabouts in Roseville and in Truckee at the Interstate 80 – Hwy 89 interchange. It makes me nervous to even watch these, and I always label these as multi-lane roundabouts to distinguish them from single-lane roundabouts or just roundabouts, which do have very significant safety outcomes.

Lastly, the type of traffic circle that engendered the discussion in San Francisco is at right. It has several non-standard features, even given the variability of traffic circles.