Here is our update about our ongoing efforts to improve traffic flow at the I-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 have resulted in increased capacity in the roundabout for afternoon drivers, we are experiencing substantial delays through the intersection for one hour on weekday mornings. New traffic count data shows that the morning traffic volume approaching the roundabout from Issaquah has increased by roughly 67 percent over the past 18 months. We understand that this congestion is frustrating to drivers. We are working reduce some of the congestion in the morning commute hour.

Construction to begin next week on a new right-turn slip lane

We expect to begin construction next week to add the right-turn slip lane from the westbound leg to the northbound leg of West Lake Sammamish Parkway. Restoring this lane will reduce congestion and delay for drivers making this turn.

A traffic-metering signal was installed last week

A new traffic-metering signal was installed last week to meter vehicles entering the roundabout from Lakemont Boulevard. The traffic meter briefly stops each vehicle from the northbound leg in order to create gaps for drivers to enter the roundabout from West Lake Sammamish Parkway. So far, the meter is working as intended, though we are still fine-tuning the duration of the red light at the meter to maximize overall efficiency of the system.

New traffic count data

Traffic counts collected this week confirm that there has been an unusually large increase in traffic volume through the West Lake Sammamish Parkway roundabout during the morning rush hour.

Traffic counts taken on the east leg of the roundabout (westbound West Lake Sammamish Parkway) in spring 2004 showed a volume of 450 vehicles per hour moving through the intersection during the morning rush hour. Roughly half of the vehicles used the right turn slip lane and half entered the roundabout. Drivers experienced only very minor delays on that leg of the roundabout, with the maximum delay of three to five minutes..

New traffic counts taken last Wednesday, September 21st, recorded a volume of 643 vehicles per hour passing through the roundabout from the westbound W. Lake Sammamish leg. Even with that volume passing through the roundabout, there was still a backup more than a mile-long at one point.

The longest delay on that leg that morning was 19 minutes. Based on those factors, the number of westbound vehicles trying to use the roundabout today is roughly 750 vehicles per hour. This equates to a 67 percent increase over the demand just 18 months ago. While, we recognize that the old configuration still had some additional capacity left in it, we are certain that the current traffic demand would have caused significant congestion on the old roundabout, as traffic backed up past the entrance to the slip lane.

Here is a summary of the traffic volume increases and performance of the new roundabout:

1) In 2004, the old roundabout processed just under 900 vehicles per hour in the evening rush hour on the southbound West Lake Sammamish Parkway approach. In 2005, the new roundabout processes over 1,000 vehicles per hour in the evening on that southbound approach.

2) In 2004, the old roundabout processed 840 vehicles per hour in the morning northbound Lakemont leg. In 2005, the new roundabout processes 925 vehicles per hour from the Lakemont leg with the meter turned on.

3) In 2004 the westbound West Lake Sammamish Parkway leg of the roundabout had a demand of, and processed 450 vehicles per hour. In 2005, the new roundabout has a demand of 750 vehicles per hour and processes 640 vehicles per hour.

We designed the new roundabout using the 2004 traffic volumes and are certain today's roundabout without the right turn slip lane would have handled that volume of traffic with only brief intermittent delays.

Traffic volume has increased throughout the area

Traffic volume has increased on each of the roads feeding the roundabout in the morning including Lakemont, Newport Way, and the westbound off-ramp from I-90. Backups through the roundabout correspond directly to the level of congestion on westbound I-90. Recent growth throughout the I-90 corridor has translated into more traffic and congestion on I-90. Morning traffic is often stop and go on I-90 between SR 900 and West Lake Sammamish Parkway.

When this occurs, we observe more drivers using the alternative routes that go through the roundabout.