An environmental assessment has been completed for a planned expansion of toll lanes along Interstate 405, which will extend toll lanes south from their current boundaries to just north of Interstate 90.

Toll lanes were completed in 2015 on I-405 stretching from Lynnwood to Bellevue as part of a plan to build out the paid lanes along the length of the notoriously congested interstate. The assessment was released by the Washington State Department of Transportation (WSDOT) on April 2.

I-405 is one of the most congested routes in Washington state with the lion’s share of congestion happening during rush hours. Drivers heading north experience heavy traffic between 6-9 a.m. on weekdays, and again heading south from 3-7 p.m. As more people continue to move to the region, traffic and congestion will increase. Between 2014 and 2016, the average weekday vehicle delay travelling the length of I-405 increased by 8 percent and transit times have increased by 12 minutes.

Toll lanes were identified as an option to help mitigate congestion by providing more road space for transit and letting drivers pay their way out of sitting in traffic.

The proposed expansion would build roughly 2.7 miles of a two-lane toll system from just north of the I-90 interchange connecting with existing toll lanes. This would involve widening the road or re-striping existing pavement to create the lanes. The existing carpool lane would be integrated into the toll lane system as well as highway widening efforts on either side south of Southeast 8th Street in Bellevue. Construction is slated to begin in 2019 and run through 2024.

In 2016, the section of highway between I-90 and Southeast 8th Street carried 206,000 vehicles per weekday. By 2025, this is expected to increase to 255,000 vehicles daily with most of the traffic volume increases occurring outside of peak travel hours. The environmental assessment wrote this will likely push drivers off the highway and overload local city streets.

High-occupancy vehicle lanes are already over capacity and don’t meet the WSDOT’s own performance guidelines, which require vehicles to be able to maintain a speed of 45 mph or more for at least 90 percent of travel time during rush hours. It has additionally degraded the reliability of buses that use the stretch of highway.

Additional improvements would be completed along with the toll lanes. These include building a rail corridor overpass adjacent to the existing structure on the Eastside Rail Corridor Regional Trail. The new structure would have two toll lanes, and the general purpose lanes would remain on the existing structure.

A new pedestrian bridge would be built over I-405 near milepost 12 that would connected to the Eastside Rail Corridor Trail as well. The Main Street Overpass, which stretches across the highway, would be reconstructed as well.

Farther to the north, the northbound I-405 and State Route 520 exit would be widened to three lanes for 600 feet beginning where the Northeast 10th Street onramp merges into the I-405 ramp.

While rules governing the tolling system haven’t been finalized, they will likely mirror toll structures seen in the northern portion of the system, including operating hours of between 5 a.m. to 7 p.m. on weekdays and vehicles with three or more people driving for free.

As with other sections of toll lanes, the revenue generated would be used to pay off construction and operation costs and other improvements on the I-405 corridor. The total I-405 corridor program involves more than 150 projects along 30 miles of interstate stretching from Tukwila to Lynnwood.

A public meeting will be held on the proposed toll lane expansion at 5:30 p.m. April 17 at Bellevue City Hall.