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