Sent: Tuesday, November 08, 2005 8:55 AM

Subject: West Lake Sammamish Parkway Roundabout--Last Update

This is our fifth and last update about our recent efforts to improve traffic flow at the Interstate 90/West Lake Sammamish Parkway Roundabout.

Background

We recently reconstructed the roundabout at West Lake Sammamish Parkway and Lakemont Boulevard near I-90 to replace the failing pavement and improve drainage. WSDOT also enlarged the size of the roundabout to improve safety and increase capacity in the roundabout. While these revisions resulted in increased capacity in the roundabout for afternoon drivers, drivers on the westbound West Lake Sammamish Parkway leg of the roundabout experienced substantial delays during the morning rush hour. We immediately responded to the problem by installing a signal on the Lakemont Boulevard leg of the roundabout to meter traffic and create gaps for drivers on the West Lake Sammamish Parkway leg to enter the roundabout. We also worked with the City of Bellevue to adjust timing of the nearby crosswalk signal. Finally, we reinstalled the right turn slip lane that allows drivers heading westbound on West Lake Sammamish Parkway to turn right (north) at the intersection without entering the roundabout.

Right-turn slip lane finish work complete this week Since the new right turn slip lane was opened to traffic on October 21st, back ups at the roundabout have nearly been eliminated. Despite a roughly 67 percent increase in traffic on this leg of the roundabout from one year ago, back-ups now rarely exceed one minute. We expect to complete the remaining work on the right turn slip lane - installing curbs and gutters - this week, though most of the work is weather-dependent. This work will take place during the day after the morning rush hour and should not disrupt traffic. Lastly, a new concrete cover will be installed in the slip lane to replace the existing metal grate as soon as the part is delivered from the manufacturer.

Long-term monitoring

Our traffic engineers will continue to periodically monitor traffic flow through the roundabout. If we determine it is necessary in the future, we are able to reactivate the traffic meter on the Lakemont Boulevard leg of the roundabout to balance the delays on each leg of the roundabout.

Continued growth expected

According to growth statistics, traffic volume through the corridor is expected to continue growing rapidly. As traffic congestion increases on I-90 drivers will increasingly use other roads, like West Lake Sammamish Parkway, as alternate routes. During morning rush hour in spring of 2004, roughly 450 vehicles per hour traveled from westbound West Lake Sammamish Parkway through the intersection, using both the roundabout and the right turn lane. Currently, morning rush hour traffic trying to enter the roundabout from westbound West Lake Sammamish Parkway has grown by roughly 67 percent, to 750 vehicles per hour. At this rate we expect the West Lake Sammamish Parkway morning traffic volumes to grow to 1,000 vehicles per hour within the next few years. Right now, there are about 2,000 vehicles per hour using the roundabout during the two to three hour morning peak period.

To try to address this growth, the legislature established funding with the 2005 gas tax increase to complete a long-range study of the Interstate 90 corridor from I-405 to North Bend. As part of this study, WSDOT will team up with cities and community groups to create short and long term strategies for reducing congestion and improving safety. Funding for this study is on hold pending the outcome of today's election.

Lorena Eng

Northwest Region Administrator

Washington State Department of Transportation