The Washington State Department of Transportation’s (WSDOT) $705 million I-405 Renton to Bellevue Widening and Express Toll Lanes project aims to relieve congestion, increase trip reliability and improve safety on one of the busiest corridors in the state. More than 200,000 vehicles drive on the corridor daily.

“The purpose is to give the public options for traveling through the corridor, the option to pay the toll with an idea of a speedier trip or stay on the general lanes,” says Gil McNabb, Engineering Manager for WSDOT. “It also helps public transit, which will be using the express lanes.”

Multiple entities have come together to make the project a reality, including King County; Sound Transit, which operates bus service along the corridor; the cities of Renton and Bellevue; and neighbors. Sound Transit plans to operate a station in the middle of the highway at Northeast 44th Street.

“It’s a great example of us working with our local partners and agencies to bring a project that will support mass transit and move more people than cars,” McNabb says. “It’s a holistic approach and is a well-integrated transportation project.”

WSDOT let the project with a design-build delivery to a joint venture between Flatiron West of Broomfield, Colorado, and Lane Construction Co. of Cheshire, Connecticut. Wood of Aberdeen, UK, serves as Flatiron-Lane’s design partner.

“There are several benefits to design-build,” McNabb says. “There is a good degree of certainty, and it reduces the owner’s risk. The other primary thing is the timeline.”

With design-build, as soon as one section is designed, contractors can start to work on it, as engineers continue work on other sections. Additionally, design-build offers the opportunity for innovation, McNabb adds, something Flatiron-Lane acted on, reconfiguring the major interchange at Northeast 44th Street.

“The biggest innovation was flipping the alignment of the interchange with 44th Street,” says Michael Van Winden, Engineer Manager for Flatiron. “We are going to build 44th underneath the new 405 bridges. It is a way to take advantage of the space we have to improve access to the work.”

The project adds 12 miles of northbound and southbound express lanes, which will operate in addition to the general purpose lanes. Currently, this stretch of highway has one high-occupancy lane, which will be combined and operate as an express toll lane. The toll rates will adjust based on congestion. The current job will connect to existing express lanes at both ends of the project.

“This is the missing link between the express toll lanes to the north and the managed lanes to the south,” says Craig Smiley, construction communications lead for the project. “When it is completed we will have a 40-mile-plus system of managed lanes.”

Flatiron-Lane is building 10 new bridges, with precast-concrete girders. Most of them are over local streets or I-405, but one spans a creek. That work is taking place on land. Five bridges will be widened and five will be retrofitted. All bridges will be upgraded to meet current seismic standards.

The project also entails interchange improvements, including a major reconfiguration of the interchange at Northeast 44th Street, which will add roundabouts on the local streets, ramps, and utility relocations.

“Roundabouts are safer for many reasons, you lower speed, collisions are significantly less, there are fewer conflict points, and they move more traffic,” Smiley says.

Southbound auxiliary general purpose lanes are being built from the Northeast 44th Street to Northeast 30th Street and between I-90 and 112th Avenue Southeast/Lake Washington Boulevard Southeast. The lanes will run from one on-ramp to the next off-ramp. It will help relieve construction. The lane will also offer a place for trucks to slow down on a hill.

Crews also will create about 10 fish culverts. Many existing culverts are used for drainage but do not have the ability to provide a way for fish to reach a body of water to spawn.

“The Sound region has hundreds of legacy culverts, and we are in the process of replacing those legacy culverts with ones that are fish-passable,” McNabb says. “It will help with salmon restoration and open up habitat to restore salmon levels in Puget sound, Lake Washington and the region.”

The project also includes Eastrail, a paved walking and bicycle trail on the west side of the interstate, between I-405 and Lake Washington, which King County funded. The trail has a 12-foot paved path, with a 6-foot gravel shoulder on one side and a 3-foot gravel shoulder on the other. There had been an existing trail, but it had to be removed to make room for the wider highway. Two of the three south segments, reaching 1.8 miles, were completed and opened to the public at the end of May 2021.

The project broke ground in 2019, and the King County Eastrail had an early completion goal, which was met.

Van Winden considers maintaining traffic through the congested corridor a major challenge. Flatiron-Lane phased the project to limit traffic disruptions.

“Minimizing impact was the number one goal for the project team,” Van Winden says. “We took as much work offline, and designed it in a way to allow the work to be built while reducing impact to the general public.”

McNabb calls building the interchanges offline a “a tall feat, it’s very impressive.”

At the Northeast 44th Street interchange, Flatiron-Lane overbuilt the permanent roadway to move all of the traffic in both directions onto the northbound lanes, when the job is complete. Direct access ramps will be constructed in the median.

Flatiron-Lane has embraced technology on this project, including GPS, total stations and other technologies for monitoring vibration and noise to remotely alert project leaders of excess vibration or noise.

The project is scheduled for completion in 2024.