Roundabouts - design & installation

A roundabout is a type of circular intersection without traffic signals or stop signs, where drivers travel counterclockwise around a center island. When entering the roundabout, drivers yield to existing traffic, then enter the intersection and exit in their desired direction.

Key characteristics

Setting/Location

Urban, Suburban, Rural, Neighborhood

Cost

Technology

Collaboration

WSDOT regions

Eastern, Northwest, Olympic, Southwest

Other names

  • Modern roundabout
  • Mini roundabout
  • Compact roundabout

Strategy description

Using yield signs and roadway curvature, a roundabout is an intersection design that reduces congestion and increases safety. They can come in a variety of shapes such as circular, oval, tear drop, peanut, or others and can be single lane or multilane.They are different from other treatments like traffic circles because they don't require stop signs or signals. They differ from neighborhood traffic calming because of their specific use at intersections.

Vehicles entering the roundabout slow down and yield to existing traffic but circulating traffic does not stop. This constant flow allows the roundabout to accommodate high volumes of traffic.

Roundabouts are a safe and efficient intersection control for all types of users, primarily because of low vehicle speeds and low number of conflict points relative to a conventional intersection type. Roundabouts have been used as an alternative to signalized intersections in other countries for many years, and are now gaining in popularity in the United States.

Key features of roundabouts include:

  • The geometric design helps control vehicle speeds, consistently limiting motorists to below 30 MPH, and typically only 15-20 MPH on entering the roundabout which enhances safety for pedestrians, bicycles, and vehicles
  • Separation of opposing vehicle directions gives a natural refuge and simplification of crossing

When to use this strategy

Creating or reconfiguring an existing signalized or stop-controlled intersection into a roundabout makes sense:

  • In addressing safety, mobility, or operational concerns observed at an existing intersection
  • When right-of-way is available or can be obtained (e.g., in an undeveloped area or where a large existing intersection is being converted)
  • While roundabouts are helpful in areas with low-to-moderate traffic volumes, they can also work for intersections with high traffic or congestion

Roundabouts work for a variety of situations depending on the needs of the intersection and project budgets.

Strategy benefits:

  • Enhances safety by slowing speeds and conflict points including left-turn against oncoming traffic, front-to-rear, and right-angle conflicts
  • Lowers the number of conflict points from 32 at a conventional intersection to 8
  • Improves pedestrian safety and mobility by providing first priority at the crossing (no waiting for a signal phase)
  • Lowers the speed differential between pedestrians, bicycles, and vehicles (vehicles traveling ~15-20 mph supports better stopping compliance and reduces risk of driver error)
  • Limits emissions and fuel consumption by reducing stop-start conditions and persistent idling
  • Reduces maintenance costs because there are no traffic signals, structures, and associated equipment to maintain

What you need in order to implement

Roundabouts require robust implementation from planning to public outreach.


Planning needs:

  • Assess adjacent development, access needs, and local policies to confirm that a roundabout makes sense in context
  • Conduct space feasibility analysis to confirm right-of-way requirements and potential obstructions
  • Conduct traffic demand analysis, including demand patterns, vehicle types, approach volumes, speeds, delay, crash history, and pedestrian and bicycle usage
  • Consider peer review of design

Operations and infrastructure needs:

  • Acquire necessary right-of-way
  • Incorporate complementary intersection improvement strategies including pedestrian treatments and access management

Outreach needs:

  • Roundabouts are often a relatively new concept to road users, collaborate with the public and ensure their concerns around access and safe operation are acknowledged and addressed
  • Use statistics, studies, videos of successful use, and other forms of evidence of roundabouts benefits to help gain buy-in and acceptance
  • Plan to provide driver education and outreach, especially if the concept is relatively unknown to the road users
  • Involve local pedestrian, bicycle, transit, and trucking/freight communities in planning and public education about roundabouts
  • Roundabouts are an opportunity to partner with community groups to incorporate local participation or public art in the roundabout design

Learn more about this strategy

National Cooperative Highway Research Program’s Report 572, Roundabouts in the United States (PDF).

Federal Highways Administration (FHWA), Roundabouts and Mini Roundabouts website.

National Highway Institute (NHI), Modern Roundabouts: Intersections Designed for Safety, a one-and-a half day, instructor-led course.

WSDOT Design Manual, Chapter 1320: Roundabouts (PDF).

WSDOT, Roundabout benefits.

Northwestern Center for Public Safety, Roundabout Design Workshop, a three-day, instructor-led workshop.
 

About key characteristics

Location notes:

Roundabouts work best for intersections with low-to-moderate speeds and lower traffic volumes. Given the sometimes larger footprint, they are a good fit for locations with a more generous right-of-way. However, these footprints have been decreasing and roundabouts can work for many physical settings.  These include neighborhood and suburban streets, rural intersections, and intersections with freeway ramps. Roundabouts may work for some urban street grids but it will depend on the intersection and grid. 

Cost notes:

There are high costs associated with right-of-way acquisition, paving, and roadway reconfiguration. Once installed, roundabouts help to avoid costs associated with traffic signal and related equipment installation. Operations and maintenance costs are low because there are limited signals, structures, or equipment to maintain.

Technology notes:

There are high costs associated with right-of-way acquisition, paving, and roadway reconfiguration. Once installed, roundabouts help to avoid costs associated with traffic signal and related equipment installation. Operations and maintenance costs are low because there are limited signals, structures, or equipment to maintain.

Collaboration notes:

Plan to collaborate with local officials, community groups, and the public when installing roundabouts. Including bicycling, pedestrian, transit, and trucking/freight communities in both planning, implementation and public outreach. Address the communities access and safety concerns. Further education may be needed to help people use the roundabout correctly.

Conditions this strategy addresses