Updated November 1, 2005
Why did we rebuild this roundabout?
We rebuilt the roundabout near I-90 at W. Lake Sammamish Blvd. between Bellevue and Issaquah because the pavement needed replacing, to help traffic from all directions move through the intersection more smoothly, and to fix a drainage problem in one section of the roundabout where water would accumulate on the roadway.
The pavement had failed so severely that we could not simply put another layer of asphalt on top. We needed to be completely dig up and replace the damaged pavement. We also needed to rebuild the roundabout with a larger center circle. Larger roundabouts have greater capacity and can serve more vehicles entering from all directions because speeds in the circulating roadway are faster and more cars can travel through the intersection at the same time.
What are we doing to address morning delays?
When the new roundabout opened to traffic, morning drivers experienced delays through the intersection and we immediately responded with a number of changes to help reduce backups:
Right-turn lane on W. Lake Sammamish Parkway
We restored a right turn lane connecting westbound and northbound W. Lake Sammamish Parkway. This turn lane lets some drivers bypass the roundabout. Morning congestion on W. Lake Sammamish Parkway has significantly improved with the reopening of this right turn lane.
Ramp meter signal
A new traffic signal began operating in late September on the south leg of the roundabout for drivers coming from Lakemont Boulevard. The signal resembles a freeway ramp meter so that vehicles coming from Lakemont Boulevard will stop briefly before entering the roundabout. The brief stop creates gaps in the stream of traffic making it easier for drivers to merge into the roundabout from the other legs of the roundabout and reducing the lengthy backups on W. Lake Sammamish Parkway. Clearly, there will still be a line up of vehicles during the busy commute hours, but traffic should flow more smoothly with less delay. The ramp meter will be turned on only when traffic volumes warrant.
Why didn't we just replace the pavement and leave the roundabout as it was?
The original roundabout was our first roundabout in the state. Since that time, approximately 68 modern roundabouts have been constructed in Washington State and we have learned much about designing roundabouts to improve safety and traffic flow. Those design improvements include:
- Enlarging the center circle. Larger roundabouts have greater capacity and can serve more vehicles on all the legs because speeds in the circulating roadway are faster and because more cars can travel in the circulating roadway at the same time.
- Increasing the angle at which the approach legs enter into the circulating roadway. The old approach legs did not smoothly direct vehicles in the direction of the circulating roadway. Instead vehicles had to make a fairly abrupt right turn as they entered the old roundabout. This element also limited the number of vehicles that could use the roundabout at once.
Why was a roundabout originally installed?
The original roundabout was built in the mid 90's to address a severe delay problem for drivers traveling westbound on W. Lake Sammamish Parkway, which was controlled using a stop sign at the time. Before the original roundabout was constructed it was becoming nearly impossible to find a gap in the north-south traffic to turn left from West Lake Sammamish Parkway and access I-90.
Why did we remove the right turn slip lanes in the new roundabout design?
The right turn lanes were removed to enlarge the center circle and stay within state-owned property. We didn't have enough money to purchase additional property. We recently reinstalled one of the turn lanes within state owned property.
While our traffic engineers were reluctant to eliminate the right turn lanes because of the additional capacity they provide, they also realized that the larger roundabout would increase capacity. Our engineers used two independent traffic modeling software tools to model the new larger roundabout without the right turn lanes versus the old smaller roundabout with the right turn lanes. The traffic volume data used for the traffic models was from spring 2004. Unfortunately, drivers are experiencing substantial morning traffic delays due to a surprising 67 percent jump in traffic volumes over the past 18 months.
How is possible that removing the right turn lanes could increase capacity in the roundabout and help traffic flow?
Traffic that enters a roundabout and then immediately turns right creates a gap in the through movement for traffic attempting to enter the roundabout from the next leg. For example, taking out the northbound to eastbound right turn slip lane forces drivers making this turn to enter the roundabout. Every time a vehicle makes this right turn from within the roundabout, it creates a gap for westbound traffic in the morning. With the right turn lane, westbound vehicles would otherwise have to find a gap into a more constant, continual stream of northbound through traffic traveling from Lakemont to Redmond.
Are there any other factors that our computer models did not take into account?
Our computer modeling did not take into account two unique aspects of this intersection:
- Traffic volumes through this intersetcion have increased a surpising 67 percent just in the past 18 months. Traffic data used in the models was from spring 2004.
- When I-90 traffic is clogged in the morning, many drivers divert to the westbound leg of W. Lake Sammamish Parkway. Traffic counts reveal that during rush hour more than 300 vehicles per hour routinely use westbound W. Lake Sammamish Parkway. This number grows to 500 vehicles per hour when I-90 traffic is congested.