June 9, 2022

The Modern Roundabout

Roundabout, Public Infrastructure, What is a roundabout, When is a roundabout best Roundabout, Public Infrastructure, What is a roundabout, When is a roundabout best

By Randy Carroll, PE, PTOE, Director, Transportation Service, Peggy Hawley, PE, Project Manager, and Ryan Betker, PE, Project Engineer 

Intersections are necessary to connect the traveling public. According to the Federal Highway Administration (FHWA), each year, roughly one-quarter of traffic fatalities and about one-half of all traffic injuries in the US are attributed to intersections. That is why intersection safety is a priority for national, state, and local municipalities and why the FHWA is committed to making them safer.

The FHWA has implemented the Proven Safety Countermeasures Initiative (PSCi) to make intersections safer. The collection of countermeasures and strategies reduces roadway fatalities and serious injuries on our nation's highways. Transportation agencies are strongly encouraged to implement these measures to accelerate the achievement of local, state, and national safety goals. Included in the suggested countermeasures is the implementation of roundabouts.

What is a roundabout?
Not all circular intersections are roundabouts. There are three main circular intersections, including neighborhood traffic circles, rotaries, and modern roundabouts. The first way to distinguish these intersections is their construction date. If built before 1990, the circular intersection is likely a rotary or neighborhood traffic circle.

Moreover, neighborhood traffic circles have raised islands. The raised islands help reduce motor vehicle speed. Rotaries have a significantly larger diameter, high-speed entries, high-speed circulating traffic, circular car weaving motions, and low capacity. In contrast, a roundabout has low entry speed, low circulating speed, yield requirements at the entry, and no car weaving motions in the circle.

What are the benefits of a roundabout?

  1. Substantially reduce crashes that result in injury and death by reducing conflict points and eliminating crossing maneuvers.
  2. Promote lower travel speeds and traffic calming by limiting approach, circulating, and departure speeds at the roundabout using raised islands and vehicle deflection.
  3. Reduce conflict points. A typical intersection with one travel lane in each direction will have 32 conflict points, while a single lane roundabout will have just eight conflict points.
  4. Improve operational performance. Incoming traffic yields to vehicles within the roundabout versus traffic stopping at stop signs or red lights to minimize lost time.
  5. Serve a wide range of traffic conditions such as intersections with high crash rates and capacity issues, new intersections with development, interchange ramps, urban locations, and rural locations.
  6. Accommodate large trucks by adding a truck apron. This paved area inside the roundabout allows large trucks' rear wheels extra room while turning.
  7. Are environmentally friendly because they reduce emissions created from idling, use less pavement than standard intersections, and require no electricity for traffic signals.
  8. Can be more aesthetically pleasing than a traditional intersection due to the reduction in pavement and integration of landscaping.

When should a roundabout be constructed?
 Consider a roundabout in any of the following conditions:

  • Intersections with high crash rates. Many municipalities consider an intersection crash rate above 1.5:1 million entering vehicles (MEV) per year to be high.
  • Intersections with capacity issues. Level of Service (LOS) is defined on an A through F scale depending on the amount of delay for the vehicle. Most municipalities will consider an intersection to have capacity issues if it functions at LOS F.
  • Unique geometry intersections. These include skewed, three-legged, and five-legged intersections.
  • Integration into reconstruction project or new design. When new projects kick off in a larger reconstruction project or future development is in the works, there are opportunities to weave in roundabouts at the beginning.

Roundabouts are one of the many solutions engineers should consider when resolving an intersection problem. However, the type of intersection control, whether a roundabout or some other type, should be evaluated on a case-by-case basis. Many state agencies and local municipalities require an Intersection Control Evaluation (ICE) to document and decide on future intersection improvements.

Westwood's certified roundabout experts have been integral in improving intersection safety with more than 80 completed roundabout projects across the United States.

Contact Westwood’s roundabout team to improve your project's intersection safety and capacity.


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Roundabout, Public Infrastructure, What is a roundabout, When is a roundabout best Roundabout, Public Infrastructure, What is a roundabout, When is a roundabout best
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