A roundabout is a type of circular intersection. Compared to signals, roundabouts tend to move traffic more efficiently, cut down on pollution, reduce the number and severity of crashes, last longer, cost less, and do not rely on electricity for operation.
A major benefit to roundabouts is that- by increasing intersection capacity- they prevent the need to widen the entire section of street. In other words, a two-lane road with roundbouts is often more efficient than a four-lane road with a traffic signals (because the traffic keeps flowing).
A study of 24 intersections in the United States that have been changed to roundabout showed an 89% reduction in fatalities and serious injury. The crashes in a roundabout tend to be 'fender benders' compared to the much more severe 'T-bone' crash typical at a signalized intersection.
Three types of circular intersections:
Traffic circle: primarily used for traffic calming in neighborhoods- to slow down traffic and prevent crashes.
Rotaries: large, fast, circular intersections. Traffic entering the circle sometimes has the right of way (opposite for roundabouts). Rotaries are not recommended because of higher-speeds, difficulty/lack of safety in maneuvering and changing lanes, and large space requirements.
*Roundabouts*: much smaller than rotaries, contain facilities for bicycles and pedestrians (in more urban areas), vehicles in the circle have right of way (over vehicles entering), entering speeds around 15mph. There is huge potential for replacing (or complementing) signals with modern roundabouts.
MIST is a strong advocate for single lane roundabouts, not for multi-lane roundabouts. (although we certainly recognize that most often a multi-lane roundabout is much better for safety and flow than a large signalized intersection).
A key area for discussion and research is the interaction between cars, bicycles and pedestrians at roundabouts. A lot of walkers and cyclists can have the effect of interrupting the flow of cars at the roundabout.
Interruptions in car flow at roundabouts due to pedestrian and bicycle activity is usually good- it slows down traffic and creates gaps for 'down stream' non-motorized (and motorized) crossings.
A concept to be explored for the U.S. is hybrid roundabout/ signal systems. The roundabout would manage traffic most of the time. The signal could help manage peak flows, emergency vehicles getting through, and sight-impaired people crossing the intersection. These hybrids are used in other countries, not so much in America. Research as of late 2015 indicates that well-designed single lane rounabouts only need proper deflection to ensure safe and calm motor speeds, and do not need any sort of traffic light or signal. Multi-lane roundabouts, on the other hand, may benefit from a signal, primarily to assist pedestrians with mobility challenges. This information is from the National Access Board, and from working groups in Missoula that are creating roundabout polity for the City.
Public Attitude Toward Roundabouts
Before And After Construction*
* taken from U.S. Department of Transportation statistics
This roundabout is on Pullen Road in Raleigh, North Carolina. The ICD- Inscribed Circle Diameter, measured from curb to curb- is 88'. Research from the Netherlands shows that this is close to the optimum size for pedestrian and bicycle safety.
This single lane roundabout in Grand Junction, Colorado, shows how bike lanes are handled. The cyclist has two options- either use the ramp up onto the sidewalk and proceed more as a pedestrian, or take the center of the lane and go around just as a motor vehicle. Bike lanes are not striped around modern roundabouts because of the risk of collisions due to blind spots.
A good roof top view of a single lane roundabout with five entering legs. The tight geometry naturally slows traffic to very safe speeds (10-15mph).
Another good view of a modern single lane roundabout. Read about the design issues here.
This roundabout is wheel chair accessible and includes a bench, landscaping and a monument in the center. Note: roundabouts typically are not designed to have human activitiy in the center, yet there are exceptions.
MIST is a great supporter of well-designed single lane roundabouts, yet does not support multi-lane roundabouts at this time because of the larger space requirements, less safety for all modes of transportation, and the tendency to not be of human scale. The above diagram shows a roundabout where a 4-lane road would meet a 2-lane road.
This is a good overhead view of a double lane roundabout (meaning two circulating lanes around the center). We advocate for measures that reduce motorized traffic so a single lane roundabout can work. This takes comprehensive planning for walking, cycling, transit and other solutions to increasing traffic.
This is a raised pedestrian crossing at a double lane roundabout. This slows cars and gives good visibility to walkers. We like this design but advocate that it be applied to single lane roundabouts near schools.
Considering Pedestrian Safety: Melbourne Australia data tells us that modern roundabouts tend to be much safer than traffic signals for pedestrian safety. Pedestrian safety is perhaps the most important benchmark for a transportation system (referenced in Missoula’s adopted growth plans).
Pedestrians at all signalized intersections (about 2,500) in the Melbourne Metro area for 2002-2006:
* Fatal: 27
* Serious Injury (hospital): 614
* Other (medical): 701
Pedestrians at roundabouts (over 4,000) in the Melbourne Metro area for 1996-2000
* Fatal: 0
* Serious Injury (hospital): 18
* Other (medical): 39
While other variables need to be considered- such as number of pedestrians and cars crossing the intersections- these statistics still illuminate the safety benefits of modern roundabouts.
Well-designed roundabouts accommodate all modes of transportation. Notice the painted bikeway slightly separated from the carway. Also notice the truck apron. This apron (the white, mountable curb area just to the outside of the inner landscaped area, and on the inside of the circulating lane) allows the back wheels of large vehicles to track over the roundabout- alleviating the need to create a larger (than necessary) roundabout for the occasional big truck. (Note: there is mounting evidence that separating bicycles and motor vehicles is not necessary with single lane roundabouts, especially if a bike slip is provided.) -mist photo from Holland, 1997.
Here is another view of a roundabout- with a bike lane striped around the outside (Denmark, 1997). Modern designs tend to not use this method- the separation can create blind spots.
Nashua New Hampshire used single lane roundabout to avoid building a 5-lane road
Is This the End of the Road for Street Lights?
Roundabout Study on reducing crashes
A newspaper article on Chatanooga roundabouts
A newspaper article on Tampa Bay roundabouts
MIST roundabout demonstration project
MIST pictures and comments for Keck Circle in Vermont
comprehensive site with pictures and links from all over the world
great site with design criteria, pictures and research
The FHWA roundabout guide
Oregon DOT Roundabout web page