Building Interstate 405
Flatiron is responsible for major upgrades along a 17-mile section of Interstate 405 in Washington designed to improve traffic flow along the busy corridor. The design-build project runs from downtown Bellevue—a tech-rich city outside of Seattle—north through the suburban communities of Kirkland, Bothell and Lynnwood.
The project includes adding additional lane in each direction along nine miles of I-405, and the conversion of an existing carpool lane into an express toll lane along the entire 17-mile project. The express toll lane facility uses dynamic tolling, which allows single drivers to use carpool lanes by paying an electronic toll, with rates based on traffic conditions.
In addition to the civil work to widen and repave the corridor, the project includes the installation of tolling infrastructure including signage, toll cabinets and conduit. Other work includes widening and retrofitting an existing bridge, adding noise walls, upgrading barriers and resurfacing much of the existing freeway. Flatiron successfully completed a major operation to restripe the majority of the 17 mile project, and turn on tolling infrastructure—all over a single weekend.
Widening and installation of express toll lanes and tolling infrastructure along 17 miles of interstate
Value engineering on braided ramp bridge led to significant reductions in retaining wall quantities and earthwork
Installation of advanced storm water treatment systems
Wetland and stream mitigation work to restore salmon habitat
A major scope of work on the project is construction of a braided ramp system that separates traffic from vehicles merging onto State Route 522 and vehicles entering I-405 from a local street. Flatiron reengineered the ramp and roadway geometry to avoid construction of a 70-foot-high retaining wall in the middle of a historic landslide area, reducing retaining wall quantities on the project by 167,000 square feet, cutting the earthwork in half.
Other value engineering and design changes included using precast concrete barriers instead of cast-in-place, and a precast bridge instead of a cast-in-place bridge, eliminating paving by reusing existing asphalt or doing an overlay instead of a full replacement.
The corridor includes new advanced storm water treatment systems, including 15,000 linear feet of new media filter drains, and, for the first time on a WSDOT project, 10,000 linear feet of compost-amended biofiltration swale. Work includes extensive wetland and stream mitigation work to restore salmon habitat in Yarrow Creek, adjacent to the project.
Flatiron closely coordinated with multiple stakeholders on the project, which included the owner, a separate tolling vendor, the cities of Bellevue, Kirkland, Bothell and Woodinville, the utility owners in each city, the metro and the tolling authority.
In addition to congestion relief, the new roadway will increase safety by reducing sideswipe and other heavy traffic-related accidents.