Elements of Access: Roundabouts

1_vancouver roundabout.png

Most people I talk with about roundabouts probably fall somewhere on the love-hate spectrum between extreme dislike and hate.  One reason for such an unenthusiastic assessment – especially for Americans – can likely be traced to some common misconceptions about what we are actually talking about when we talk about roundabouts.

Having grown up near Boston, I spent countless Friday evenings on hot summer nights sitting in traffic with everyone else trying to get to Cape Cod.  The Cape Cod Canal cuts the area off from the rest of Massachusetts, so the only way across by car meant traversing one of the two bridges.  For years, both bridges also had what those from New England called a “rotary” on one side or the other.  Traffic would routinely back up for miles at both bridges, making what could have been a 75-minute drive considerably longer.  The culprit was often these multilane rotaries.  Such intersections proved to be inefficient – both in terms of traffic flow and land consumption – as well as dangerous.  You may also remember Clark Griswold getting stuck in a London traffic circle in the 1985 movie European Vacation. “Hey look kids, there’s Big Ben!  Parliament!” Homer Simpson was in a similar situation in a 2003 episode and nearly killed the Queen of England!  Examples like these end up giving all circular intersections a bad rap.  Not surprisingly, circular intersections were essentially phased out of most U.S. design toolboxes and the minds of many Americans.

2_sagamore rotary.png

Modern roundabouts, not developed until the 1960s, refer to something quite different.  For one thing, roundabouts are much smaller than the old rotary intersections.  Instead of outside diameters exceeding 300’ or 400’, modern single-lane roundabouts typically range between 90’ and 180’.  Another thing is that the cars entering must yield to the cars already in the roundabout (although this was usually, but not always, the case with the older traffic circles and rotaries).  The main defining characteristic of modern roundabouts, however, has to do with speed deflection.  Speed deflection refers to angle at which cars enter the roundabout.   With the old rotaries, there was little to no horizontal deflection of through traffic so cars could easily exceed 30 mph.  A well-designed modern roundabout typically has enough deflection in the angle of this approach to actively manage vehicle speeds to less than 20 mph.  It can also still handle truck traffic with design features such as a traversable apron that skirts the inner circle, which can be seen in the top image from Vancouver.

Elements of Access: Transport Planning for Engineers, Transport Engineering for Planners. By David M. Levinson, Wes Marshall, Kay Axhausen.
Elements of Access: Transport Planning for Engineers, Transport Engineering for Planners. By David M. Levinson, Wes Marshall, Kay Axhausen.

So what does the research tell us about modern roundabouts?  In most contexts, they move traffic more efficiently and are safer than conventional intersections.  Why would this be the case?  In terms of efficiency, there is no waiting for the light to turn green when there is no cross traffic.  In fact, single-lane roundabouts have been shown to reduce delays as compared to conventional intersections and effectively manage traffic flows as high as 25,000 cars per day.   Less idling also means fewer emissions.   In terms of safety, roundabouts eliminate conflict points and the most dangerous types of conventional intersection crashes; while you may get more sideswipe or rear-end crashes, such crashes are far less likely to be fatal or severe injury.  Also if the roundabout is designed with adequate deflection, these crashes tend to happen at slower speeds.  This reduces crash severity to the tune of 78-82% fewer serious injury or fatality crashes as compared to conventional intersections (AASHTO Highway Safety Manual).

There are valid concerns about pedestrians and bicyclists in roundabouts, but splitter islands, setback crosswalks, and sidewalks – when combined with slower vehicle speeds – help tremendously.  Interestingly, many places allow bicyclists to act as either a vehicle or a pedestrian in roundabouts.  My own concerns center more on effectively serving those with impaired vision, which is still an issue with most roundabouts.

While I would argue that multilane roundabouts are unnecessarily used in many situations where a one-lane roundabout would work well, multilane roundabouts still offer many of the same advantages.  Compared to single-lane roundabouts, however, they do: lose some speed deflection when volumes are low; introduce a new crash type to the mix (i.e. sideswipe crashes due to lane changes); and make things more difficult for pedestrians and bicyclists. You can also include neighborhood traffic circles – which are even smaller than most modern roundabouts – in this overall discussion of circular intersections.  The example below from Berkeley, CA combines 4-way stop control with a circular intersection.  While not quite a roundabout, it is a good example of using a small traffic circle to help manage speeds and improve safety.

3_berkeley traffic circle.png

Compared to signalized intersections, roundabouts are generally less expensive, more efficient, more environmentally friendly, and perhaps most importantly, safer.   Furthermore, you never have to worry about a power outage with a roundabout.  While there are legitimate reasons not to use roundabouts in some situations – such as highly unbalanced traffic flows or ROW limitations – many get eliminated as an option due to our cultural biases against them.  All we are saying is give roundabouts a chance.