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East Lake Sammamish Parkway/SE 43rd Way Roundabout, City of Issaquah, Washington: Three-leg, multi-lane, open 2010

As the main access route from I-90 into the city of Sammamish and the intersection of two trails–the East Lake Sammamish Tail and Laughing Jacob’s Creek Trail–the intersection of East Lake Sammamish Parkway (ELSP) and Southeast 43rd Way was a congested intersection for commuters, bicyclists, and pedestrians.  Due to its location at the borders of the cities of Issaquah and Sammamish, the intersection had to be highly efficient and provide an attractive gateway into both cities.

The initial feasibility study was taken on by the city of Sammamish who selected Reid Middleton to prepare an independent intersection analysis comparing traffic signals and roundabouts.  Led by our resident roundabout expert, Patrick McGrady, the analysis demonstrated that a 3-leg, hybrid roundabout with two bypass (slip) lanes vastly out-performed the original traffic signal system while providing enough reserve capacity for future traffic growth beyond 2030.

Before and after Google imagery of the intersection

The project was driven by a development project on the northeast quadrant of the intersection.  Through a series of events, the final design and construction phase of the project was taken over by the city of Issaquah who hired us to develop the roundabout final configuration and the construction plans.
The hybrid roundabout design exemplifies what can be engineered to accommodate specific traffic pattern requirements within a constrained site.  A portion of the circulating lane is two lanes, while the rest is one lane.  The roundabout also features one and two-lane entrances and exits as well as two bypass lanes.

East Lake Sammamish Parkway and SE 43rd Way roundabout – open 2010

The roundabout also experiences a high volume of commuter bicycle traffic.  At the 2011 Annual Meeting for the Transportation Research Board (TRB), a video showing the diverse mix of bicyclists and motor vehicles traversing through this roundabout in harmony was highlighted at the Roundabout Video Theater.  This video, found at:

shows several examples of bicyclists comfortably claiming the lane in line with motor vehicles during the evening peak hour. While most bicyclist choose the travelling lanes, viewers will see one cyclist who chose to take the bike ramp, multi-use path and cross walk system provided.  Note that the roundabout geometry slows motor vehicle traffic to bicycle speeds–a highly desirable feature in roundabout design.

Bicyclist using the roundabout

To continue the conversation about roundabout design, leave a comment on our blog or send us an email.

2 Comments

  1. Please explain why it is “highly desirable” for cyclists to have to slow down to motor vehicle speeds in a roundabout. I can see it desirable that vehicles pass through at same speed, but as a cyclist it’s a bother when the cars don’t get out of the way!

    Reply
    1. The statement you are referring to was not worded well and has been changed. The “highly desirable” feature of roundabout design is how the geometry slows motor vehicles to bicycle speeds. This allows the commuter bicyclists to comfortably merge with the circulating motor vehicles. When the traffic is heavier during the morning and evening peak hours, the bicycles using the circulating lane will experience more delay as do the motor vehicles. Note that these peak hour delays are much less than with the previous traffic signal.

      Thank you for your comment.

      Reply
  2. I’m not sure that your statement ” shows several examples of bicyclists comfortably claiming the lane in line with motor vehicles during the evening peak hour ” is highly accurate. I ride north on E L Sammamish Parkway and that is one of the most dangerous intersections because cars will not slowdown until they get into the circle. I’ve had so many close calls at that intersection from cars that are trying to pass me while I get into the travelling lane that I’m considering hopping onto the gravel trail for that section. Firstly, there should be a painted bicycle lane all along that section of E Lake Samm Pkwy. Second, it would be beneficial to both cyclist and motorists if there was a painted bicycle lane that leads onto the travelling lane(just like the one that heads south on E L Sammamish Pkway that goes east onto Inglewood.

    Reply

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