Modern Roundabouts

What is a Modern Roundabout 

A modern roundabout is a circular intersection where drivers travel around a center island that promotes slow and consistent vehicle speeds entering, circulating, and exiting the intersection in a one-way direction. Drivers entering the roundabout yield to traffic already in the roundabout, then enter the circulating roadway and exit at their desired street. 

History of Roundabouts

Modern roundabouts differ from previously constructed traffic circles and rotaries with their smaller size, slower speeds, and lack of lane changes within the intersection. The initial circular shaped intersection goes back to the 1790s when Pierre Charles L’Enfant, an architect/engineer proposed several circular intersections when designing streets for Washington DC, including the famous Dupont Circle. In 1905, the form was revived for New York’s Columbus Circle which is considered to be the first traffic circle intersection in the automobile era.

In the 1930s Americans began building rotaries.  Like the modern roundabout, the intersections featured splitter islands to separate entering and existing lanes, giving the entrance into the rotary a more gradual angle. The main difference between rotaries and the modern-day roundabout is that traffic in the circle was required to yield to those entering the circle.  This led to congestion and high collision rates causing the design to fall out of favor in the US in the 1950s.

In 1966, the British changed the yield rule and those that were entering the circle had to yield to those already in the circle.  They found that this increased mobility through the rotary and that vehicle crashes decreased by 40%.  This gave birth to the modern roundabout. 

Over the next few decades, the British concept spread throughout Europe, and the United States constructed the first modern roundabout in Nevada in 1990.  Modern roundabouts continue to become more popular due to the increase in safety and mobility through intersections. The modern roundabout design continues to evolve to meet the needs of our ever-changing transportation system. 

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How to Navigate Roundabouts 

Modern roundabouts are designed to accommodate vehicles of all sizes, including emergency vehicles, buses, and truck and trailer combinations. In a modern roundabout, drivers enter the intersection by navigating a gentle curve. Drivers entering the roundabout yield to traffic already in the roundabout, then proceed into the intersection and exit at their desired street.

A main feature of the modern roundabout is a raised central island. The circular shape is designed to control the direction of traffic and reduce speeds to 15 to 20 mph. It also reduces the likelihood of T-bone (right angle) or head-on collisions.

The central island of many roundabouts includes a truck apron, which is a raised section of concrete that acts as an extra lane for large vehicles. The back wheels of the oversize vehicle can ride up on the truck apron so the truck can easily complete the turn, while the raised portion of concrete discourages use by smaller vehicles. In multi-lane roundabouts, large vehicles may straddle both lanes to make their turn. Because the rear of the vehicle or trailer is likely to off-track into the other lane while making a turn, other drivers should never drive next to large vehicles in a roundabout.

Roundabouts Provide Safety and Mobility Benefits

Roundabouts can provide substantial safety and mobility benefits compared with conventional intersections. Contrary to many peoples' perceptions, roundabouts move traffic through an intersection more quickly, with less congestion on approaching roads by promoting continuous flow of traffic. Unlike intersections with traffic signals, drivers don’t have to wait for a green light to get through the intersection. Traffic is not required to stop – only yield – so the intersection can handle more traffic in the same amount of time. This allows for a smoother, more efficient flow of traffic.

Roundabouts are safer than traditional traffic signals or all-way stop-controlled intersections because they have fewer conflict points between turning vehicles, through traffic, and pedestrians crossing. The tight roundabout circles and curbed median islands promote slow driving speeds which give drivers more time to react to people in crosswalks or other vehicles. The slower vehicle speeds also result in less severe crashes if they do occur.

In addition to having fewer serious conflicts between vehicles than traditional intersections, roundabouts are generally safer for pedestrians as well. In a roundabout, pedestrians walk on sidewalks around the perimeter of the circular roadway. If they need to cross the roadway, they cross only one direction of traffic at a time. In addition, crossing distances are relatively short, and vehicle speeds tend to be low.

The Future of Roundabouts in Arlington 

Modern roundabouts are becoming the future of intersections, this is evident throughout Washington and in the Arlington area.  The rise in popularity of this intersection control is due to improved safety, reduced collisions, and improved traffic flow.  The Washington State Department of Transportation built its first roundabout in our area in 2011 at the intersection of SR-9 and SR-531. With the recent completion of two new roundabouts along SR 530, the Arlington area currently has seven roundabouts with many more planned in the future. 

Why is Arlington planning for more roundabouts? When compared to traditional traffic control there are several facts and figures on the improved safety and increased traffic flow of roundabouts. The Federal Highway Administration and the Insurance Institute for Highway Safety have completed studies showing the installation of roundabouts resulted in a 40 percent reduction in pedestrian collisions, 37 percent reduction in overall collisions, 90 percent reduction in fatality crashes and 75 percent reduction in injury collisions. Another study completed for Washington State showed an 89 percent decrease in traffic delays and 56 percent reduction in motor vehicle stops. 

What does this mean for Arlington drivers?  This means fewer red lights, less traffic delays, less chance of accidents, and less chance of severe injuries.  You can be part of this future by participating in the planning and design of future roundabouts.  The City’s Public Works Department looks to partner with the Arlington community with the planning and layout of future roundabouts, sign up to receive Arlington’s weekly Newsletter and look for upcoming public meetings on roundabout project planning and design.

Roundabout Interactive Map

If you would like a full screen version of the below map, please click here. 

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Key Things to Remember About Driving Through Roundabouts:

  • Slow down as you enter and travel through the roundabout. 
  • Yield to pedestrians in the crosswalk.
  • Yield to drivers already in the roundabout (in the circulating roadway).
  • Stay in your lane; do not change lanes.
  • Do not stop in the roundabout (in the circulating roadway).
  • Avoid driving next to oversized vehicles. 

Benefits

Traffic Flow Benefits:

  • Decreases in vehicle delays.
  • Reduction in vehicle stops.
  • Faster commute times.
  • Increases intersection capacity.
  • Reduces noise caused by braking hard and accelerating at intersections.

Vehicle Safety Benefits:

  • Fewer conflict points between vehicles in an intersection.
  • The most severe types of vehicle-on-vehicle crashes, such as T-bone, left-turn, and head-on collisions are avoided.
  • The roundabout central island and traffic medians force drivers to slow down to between 15 and 25 MPH.
  • Slower vehicle speeds reduce the stopping distance needed which helps drivers avoid crashes.
  • Slower vehicles speeds lessen the chance for fatal and serious injury crashes.
  • Reduces injury crashes by 75% where stops signs or traffic signals were previously.

Pedestrian Safety Benefits:

  • Slower vehicle speeds help drivers recognize people trying to cross the road.
  • People walking cross one stream of traffic at a time. People walking can pause between the traffic entering the roundabout and exiting the roundabout.
  • Roundabouts reduce the conflict points between people biking and vehicles.
  • Rapid Rectangular Flashing Beacons (RRDBs) are used to signal vehicles of pedestrians.