Last Update: June 2020
What’s to like about roundabouts?
PLENTY. As states, counties, and cities are continually challenged to maximize traffic safety and efficiency, roundabouts have started showing up throughout our region. In fact, given their many benefits, State and Shoreline guidelines require that roundabouts be analyzed as an option to standard signalized intersections with any new intersection improvement project.
One of the best advantages of a roundabout is that they significantly reduce serious injury collisions. Roundabouts accomplish this through their geometry:
- Curved lanes and one-way travel basically eliminate head-on and right angle (T-bone) contact.
- Roundabouts also have lower design speeds, typically 15-20 mph, so if a collision does occur, it tends to be minor.
- Potential conflict points are fewer. A single lane roundabout has 8 vehicle and 8 pedestrian conflict points compared to 32 vehicle and 24 pedestrian conflict points at a comparable signalized intersection. A multi-lane roundabout would have additional conflict points and may have designated movements for certain lanes, but they still include most of the benefits of single-lane roundabouts and contain fewer conflict points than a comparably sized signalized intersection.
|Roundabout vehicle and pedestrian
| Signalized intersection vehicle and pedestrian
|Image Source: AARP|
There are additional benefits all roundabouts have in common:
- Efficient. Reduction in travel delays and vehicle stops.
- Greener. Less idling and stopping/starting means less fuel consumption.
- Cost effective. Though generally similar initial construction costs, there are no ongoing signal maintenance and electrical costs.
- Pedestrian safety. Crosswalks are set back from the vehicle intersection simplifying the driver and pedestrian task by allowing drivers and pedestrians to focus on one conflict point and one direction at a time.
- RRFBs (rectangular rapid flash beacons) can be added for additional visibility, with bright flashing lights indicating the presence of a pedestrian. RRFBs are pedestrian activated.
|Image illustrates how crosswalks are set back from the roundabout lanes.
RRFBs indicate the presence of a pedestrian and are visible both day and night.
Image Source: Google Maps
Modern roundabouts are very different from small neighborhood traffic calming circles or older rotaries common to the east coast and parts of Europe. To read more about these differences, visit the Washington State Department of Transportation webpage at: https://www.wsdot.wa.gov/Safety/roundabouts/BasicFacts.htm
Where will Shoreline see its first roundabouts?
Shoreline welcomed its first roundabout at NE 185th Street and 10th Avenue NE. This compact, urban roundabout was installed by Sound Transit to help with traffic movement during the closure of 5th Avenue NE and light rail construction.
Just down the street from this roundabout will be a second roundabout at NE 185th Street/8th Avenue NE which will be constructed as part of the Sound Transit light rail project in order to address trips to/from a new parking garage at the Shoreline North/185th Station which opens in 2024.
Two current projects will also include or are considering roundabouts:
145th / I-5 Interchange. WSDOT (Washington State Department of Transportation) has agreed with the design and construction of two roundabouts on each side of the I-5 overpass on 145th Street. See the Interchange Project webpage for additional information on this project.
160th and Greenwood / Innis Arden (Shoreline Community College). Improvements are being made to the intersection at the entrance to Shoreline Community College and one of two potential options being considered is a roundabout. Follow the progress on the project webpage.
Roundabouts will be considered for any future intersection improvements along the 175th Street and 185th Street corridors as well as other sites throughout the City if an intersection is redeveloped.
A “roundabout rodeo” is a proven method for showing the functionality of a proposed roundabout design. For the Interchange Project, the design was laid-out to scale based on engineering drawings and then Metro bus drivers were sent through to make sure they could navigate the entrances, radius, and exits properly. The WSDOT Regional Transit Coordination Division assisted King County Metro in organizing this exercise with Sound Transit, Seattle Department of Transportation, and the City of Shoreline.
|Video captures collaboration of event.|