Tools for sustainable transportation

Roundabouts: circular intersections for safety and flow

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.

Another benefit of roundabouts is that they often prevent the need to widen an entire section of street. In other words, a two-lane street with roundabouts is typically more efficient than a four-lane street with traffic signals.

A study of 24 intersections in the United States that have been changed to roundabouts showed an 89% reduction in fatalities and serious injury. Any crashes that occur in a roundabout tend to be 'fender benders' compared to the much more severe 'T-bone' crash common at signalized intersections.

Three types of circular intersections:

Traffic circle: primarily used for traffic calming in neighborhoods- to slow down traffic and prevent crashes.

Rotary: 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, lack of safety in maneuvering and changing lanes, and large space requirements. In fact, rotaries are not being built any more and are often converted to modern roundabout.

Roundabout: much smaller than rotaries, includes facilities for bicycles and pedestrians (in more urban areas), vehicles in the circle have right of way over vehicles entering, entering speeds range from 8mph to 17mph depending on the angle of deflection in the design. There is huge potential for replacing (or complementing) signals with modern roundabouts.

MIST is a strong advocate for modern, accessible, single lane roundabouts. We are not supportive of multi-lane roundabouts. While we recognize that most often a multi-lane roundabout is much better for safety and flow than a large signalized intersection, our work focuses on reducing the need for multi-lane streets in urbanized areas. Without multiple travel lanes in the same direction, there is little need for more than one circulating lane in a roundabout.

A key area for continued discussion and research is the interaction between cars, bicycles and pedestrians at roundabouts. People walking have the right of way, at the crosswalks. The crosswalks are set back one car length from the circulating lane and move through the splitter island. The best design for people on bikes includes protected bike lanes between the circulating lane for motor vehicles and the pedestrian way/ sidewalk. With a single lane roundabout, it is generally safe and easy for a cyclist to also go through the roundabout in the same circulating lane used by motor vehicles.

A concept and design to be further explored is the hybrid roundabout/ signal system. The roundabout manages traffic most of the time, while various forms of signals help manage peak flows, emergency vehicle access, and pedestrian crossings by those wheel chairs, canes and service dogs.

Research indicates that well-designed single lane roundabouts only need proper geometric design 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, can benefit from a signal, primarily to assist pedestrians with mobility challenges. MIST participates in working groups in Missoula that are creating roundabout policy for the City.

Here are some recent U.S. Access Board recommendations to make roundabouts accessible to pedestrians with mobility challenges:

• single lane crossings at entrance and exit*

• raised crossings , especially at exit 

• 'YIELD-TO-PED'  markings

• pedestrian lighting

  *we believe that ’single lane’ is the most important aspect of safe and accessible roundabout design
  
  
  Good size and scale for a single lane roundabout:

The above 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.

Bike lane at roundabout:

The single lane roundabout, above, is in Grand Junction, Colorado, and shows how bike lanes are incorporated on the approach. The cyclist has two options- either use the ramp up onto the sidewalk/path and proceed more as a pedestrian, or take the center of the lane and go around similar to driver. Bike lanes are not striped around modern roundabouts in the circulating lane because of the risk of collisions due to blind spots.

Rooftop view of a single lane roundabout:

The above single lane roundabout has five entering legs. The tight geometry naturally slows traffic to a safer speed in the 10-15mph range. However, there is a fast exit design with a shallow angle of deflection, on the exiting leg nearest in the picture. MIST advocates and promotes tighter geometry to facilitate slower speeds and smoother flow, for all.

Example of single lane roundabout with fast exit design (bad urban design):

Below is an overhead view of a multi-lane roundabout. While MIST supports well-designed single lane roundabouts, we believe a community should take measures to reduce auto traffic so that multi-lane roundabouts are not needed, especially in urban areas. Compared to single-lane roundabouts, multi-lane roundabouts have larger space requirements, are less safe for all modes of transportation, cost more and tend to not be of human scale.

Double lane roundabouts tend to be less safe for all modes, although injuries are still rare when compared to traffic signals. Still, we advocate for measures that reduce motorized traffic so a single lane roundabout can work. This takes comprehensive planning for walking, cycling, transit and land use. A single lane roundabout can move about 25,000 cars a day, and sometimes much more. The key is the peak hour traffic. If a community can shift peak hour/ rush hour traffic to other hours of the day, then single lane roundabouts can be even more effective. Our work focuses on shifting routes, times and modes, so that modern single lane roundabouts can be utilized on all City arterial streets.

Public Attitude Toward Roundabouts Before And After Construction*

Attitude Before Construction After Construction
Very Negative 23% 0%
Negative 45% 0%
Neutral 18% 27%
Positive 14% 41%
Very Positive 0% 32%

* U.S. Department of Transportation statistics

Visualizing Roundabouts, Circular Flow and Changes to Streets: The below picture is of MIST staff and volunteers setting up a demonstration roundabout in Missoula, Montana. We set the roundabout up in 10 minutes, using green materials all brought to the intersection in a bike trailer. The inner circle and splitter islands are scraps of wood planking, the green in the circle is a bag of cut lawn trimmings and we borrowed a single flower pot from a neighboring business to place in the center. Immediately, traffic disregarded the traffic signal and intuitively utilized the roundabout.

Roundabouts can help transform a city by increasing safety and flow while also saving money and pollution. Communities should strive to make single lane roundabouts work where possible and avoid double lane roundabouts. This effort takes community planning and advocates for sustainable transportation.