Roundabouts

 

What is a “Modern Roundabout”?

A roundabout is a one-way, non-signalized circular intersection in which traffic flows around a center island. Roundabouts have specific design and traffic control features ensuring low travel speeds and efficient traffic movement, while meeting the needs of all road users, including passenger cars, large trucks, buses, pedestrians, and bicyclists. A roundabout is a unique traffic device, different from a Traffic Circle, Circulator, Rotary, or other intersection.

How does a Roundabout differ from a Traditional Intersection?

A roundabout reduces vehicle-to-vehicle conflict points from 32 (at a conventional four-way intersection) to 8 (See figure below). Among those conflict points eliminated are left-hand turns, which can result in more serious crashes at traditional intersections. Since roundabout traffic only enters and exits via right-hand turns, occurrence of severe crashes is greatly reduced. Small angle collisions may occur as a result of right-hand turns, but these are typically less severe types of collisions. According to the Federal Highway Administration (FHWA) Office of Safety, roundabouts have a greater than 90% reduction in fatalities, a 76% reduction in injuries, and a 35% reduction in all crashes when compared to conventional intersections.

How does a “Roundabout” differ from a “Rotary” or “Traffic Circle”?

Roundabouts are not Traffic circles. Traffic Circles (or rotaries) operate under different traffic rules and have experienced operational and safety problems. Older style traffic circles (such as those found in Boston, Washington, D.C., Paris, and London) are significantly larger than roundabouts, and were designed for high speed entry and multi-lane weaving. These traffic circles generally suffered high crash rates and operational problems causing many to fall out of favor in the US during the 1950s and 1960s. Roundabouts are commonly referred to as “Modern Roundabouts” to distinguish them from these older style traffic circles (Click here for more details).


Source: FHWA ~ Office of Safety

How does a Modern Roundabout improve upon these Safety Concerns?

Many traffic circles require circulating vehicles to grant the right of way to entering vehicles, whereas modern roundabouts generally follow the “Yield-to-Left” rule where vehicles in the roundabout would have the Right-of-Way.

There are three safety design features of a roundabout;

  • 1. Yield Control of entering traffic.
  • 2. Channelized Approaches that deflect traffic into the proper one-way, counterclockwise flow.
  • 3. Geometric Curvature of the circular road and angles of entry to slow the speed of vehicles.

These three features are critical to the success of a roundabout because they effectively decrease driving speed to 15-25 miles per hour (-Transportation Research Board ~ NCHRP Report 572, 2007).

Are there examples of Modern Roundabouts in Vermont?

Yes… There is a modern roundabout in Montpelier, Vermont (otherwise known as Keck Circle). Built in 1995 (on VT State Route 12), Keck Circle is Vermont’s first modern roundabout. It is also the first modern roundabout constructed on a State Highway within the United States. Since its construction, this 13,000 vehicle per day intersection has had a 69% reduction in injury accidents, and only 4 reportable accidents within the first 10 years of its operation (-NE Roundabouts, 2008).

Can large Trucks or Buses use Roundabouts?

Yes… Modern roundabouts are designed for the largest vehicle reasonably anticipated and, if necessary, are constructed with mountable curbs and concrete “Aprons” which are designed to be driven over by large trucks or buses. The truck apron generally is composed of a different material texture than the paved surface, such as brick or cobble stones, to discourage routine use by smaller vehicles.

Will Emergency Vehicles such as Fire Trucks and Ambulances be able to use Roundabouts?

Yes… These larger vehicles are taken into consideration when designing and constructing a roundabout. While the radius of a roundabout may seem too small, they are designed so that larger vehicles can drive over the truck apron (between the circulatory roadway and the central island) if necessary. Just as they are required to do at conventional intersections, drivers should be educated not to enter a roundabout when an emergency vehicle is approaching on another leg. Once having entered, they should clear out of the circulatory roadway if possible, facilitating queue clearance in front of the emergency vehicle.

What about School Buses and Snowplows?

Yes… All types of vehicles are taken into consideration when designing and constructing modern roundabouts. Since roundabouts are designed to slow all traffic when entering and eliminate left-hand turns, safety is improved over traditional intersections.

Do Roundabouts take up a lot of space?

No… Roundabouts are designed to take up as little space as necessary while meeting engineering design standards. An approximate rotary diameter is greater than 600 ft. while an approximate roundabout diameter is less than 300 ft. In some cases, roundabouts can take up less space than a traditional intersection. Landscaping in the center of a roundabout can help minimize visual impacts and help to incorporate the facility into the surroundings. Roundabouts can also provide an attractive gateway into a community.

How do Pedestrians use a Roundabout?

Roundabouts generally are safer for pedestrians than traditional intersections. In a roundabout, pedestrians walk on sidewalks around the perimeter of the circulatory roadway. If it is necessary for pedestrians to cross the roadway, they cross only one direction of traffic at a time. Additionally, crossing distances are relatively short, especially where “Splitter Islands” provide refuge for pedestrians where crossing is permitted, and traffic speeds are lower than at traditional intersections. Roundabout design discourages pedestrians from crossing to the center island.

How do Bicycles use a Roundabout?

Bicyclists have a couple of options when approaching a roundabout. They may take the full travel lane and follow the same yield-to-left rule as a vehicle. Since traffic speeds are slower at a roundabout, bicycles fit into the normal traffic flow. Bicyclists may also choose to dismount at the roundabout and use the sidewalks and crosswalks to navigate to their desired exit point as a pedestrian.

How much Traffic can a Roundabout handle?

Single-lane roundabouts can typically handle between 20,000 – 26,000 vehicles per day, depending on the number of left-turning vehicles and the distribution of traffic between the major and minor roads.

In general, roundabouts provide greater capacity (accommodate more vehicles) than all way stop control intersections and reduce delays compared to signalized or all-way stop control intersections. The reduced delay of roundabouts is generally attributed to:

  • 1. Continuous flow of traffic, which reduces start and stop delay;
  • 2. All right-turn movements, which are more efficient and require shorter gaps in traffic;
  • 3. Lower operating speeds, which further reduce gap requirements;

Roundabouts with more than one lane can handle greater traffic volumes. The references below provide more detailed information on roundabouts compared with other traffic control alternatives.

Are Roundabouts dangerous?

No… Good roundabout design results in slower traffic speeds. Some potential safety benefits of slower traffic speeds in roundabouts include:

  • 1. Reduce crash severity for pedestrians and bicyclists, including older pedestrians, children, and impaired persons;
  • 2. More time for entering drivers to judge, adjust speed for, and enter a gap in circulating traffic;
  • 3. Allowance for safer merges into circulating traffic;
  • 4. More time for all users to detect and correct for their mistakes or mistakes of others;
  • 5. Less frequent and less severe collisions; and
  • 6. More safety for novice users.

Additionally, roundabouts are safe when the power is interrupted. Where traditional intersections can become a safety concern during outages (especially in higher volume intersections), roundabouts need electricity only for nighttime illumination, requiring a slight reduction of driver speed to retain their level of operation.

Does the State of Vermont require roundabout installation at dangerous intersections?

No… Roundabouts are usually considered as one of several options for traffic control at intersections. The Vermont legislature passed
Act 141 in 2002, which included a section requiring consideration of roundabouts at intersections with high crash rates.

“The general assembly finds that the installation of roundabouts at dangerous intersections in the state has been cost-efficient, and has enhanced the safe operation of vehicles at these locations. The agency of transportation is directed to carefully examine and pursue the opportunities for construction of roundabouts at intersections determined to pose safety hazards for motorists.” (H. 764, Sec. 37, 21 Jun 2002)

State and/or municipal project development processes then evaluate all of the alternatives under consideration before advancing a design selection for construction. More information on the project development process is available in the VTrans Project Development Process Manual.


Visit these sites for information about Roundabouts, including more FAQs, photos, videos, and more:

State & Local Educational Resources

National Information

© 2011 CCRPC Chittenden County Regional Planning Commission
110 West Canal Street, Suite 202
Winooski, Vermont 05404-2109