Interstate 405 (Washington)

Interstate 405 (I-405) is a north–south auxiliary Interstate Highway serving the Seattle region of Washington, United States. It bypasses Seattle east of Lake Washington, traveling through the Eastside area of King and Snohomish counties, providing an alternate route to I-5. The 30-mile (48 km) freeway serves the cities of Renton, Bellevue, Kirkland, and Bothell. I-405 terminates at I-5 in Tukwila and Lynnwood, and also intersects several major highways, including SR 167, I-90, SR 520, and SR 522.

The Eastside highway was originally built in the early 20th century to connect cities along the lake and was formally added to the state highway system in 1937 as Secondary State Highway 2A (SSH 2A). A freeway replacement for SSH 2A was proposed in the 1940s by the state government and designated as I-405 as part of the federal Interstate Highway program, with the first section beginning construction in 1956 and completed in 1965. It was initially signed as SR 405 until the freeway was fully completed in 1971; since then, the highway has been expanded to add lanes for high-occupancy vehicles and toll users. I-405 is one of the most congested highways in the Seattle area and is known for its meandering "S-curves" through Renton, which were straightened in the 1990s.

I-405 is a 30-mile (48 km) north–south freeway that serves as a bypass of I-5 through Seattle while serving the Eastside region. It is listed as part of the National Highway System, identifying routes that are important to the national economy, defense, and mobility, and the state's Highway of Statewide Significance program, recognizing its connection to major communities. The highway is maintained by the Washington State Department of Transportation (WSDOT), who conduct an annual survey of traffic volume that is expressed in terms of annual average daily traffic (AADT), a measure of traffic volume for any average day of the year. Average daily traffic volumes on I-405 in 2016 ranged from a minimum of 76,000 at its southern terminus in Tukwila to 209,000 in Downtown Bellevue. Approximately 86 percent of peak-direction lane miles on I-405 are rated as "routinely congested" by WSDOT and the highway accounted for over 30 percent of delays on the Seattle area's urban freeways from 2013 to 2017.

The freeway has a system of high-occupancy vehicle lanes (HOV lanes) from Tukwila to Downtown Bellevue that become high-occupancy toll lanes (HOT lanes) from Bellevue to Lynnwood. The HOT lanes are controlled through a series of designated access points and direct ramps located along I-405. Tolls are collected electronically through Good to Go transponders by overhead sensors or via license plate cameras for mail billing with a surcharge. The variable weekday toll rates are set according to traffic congestion, ranging from $0.75 to a maximum of $10, while weekends and federal holidays are toll-free. HOT lane tolls are waived for high-occupancy vehicles with three or more passengers during peak periods and two or more passengers during the mid-day when using the "FlexPass" that can toggle between tolled and HOV modes.

I-405 begins as a continuation of State Route 518 (SR 518) at a multi-level junction with I-5 in Tukwila, northeast of Seattle–Tacoma International Airport. The eight-lane freeway gains a set of HOV lanes, directly connected to I-5, and travels around the north side of the Southcenter Mall, at the center of Tukwila's retail district. The mall is served by a set of auxiliary ramps around the periphery of the I-5/SR 518 interchange. I-405 travels east across the Green River and intersects Interurban Avenue at the north end of SR 181, located near the Tukwila train station and Starfire Sports soccer complex at Fort Dent Park. After crossing a set of railroad tracks (part of the BNSF Railway's Seattle Subdivision and the Union Pacific Railroad), the freeway enters the city of Renton and passes between a regional wastewater treatment plant and the former Longacres racetrack (now a Boeing office park). I-405 continues across the Black River and through an industrial and commercial area on the southern outskirts of Renton to a cloverleaf interchange with SR 167 (the Valley Freeway) and Rainier Avenue. The interchange, located near several car dealerships at the northwest corner of Talbot Hill, includes a direct HOV flyover ramp from I-405 southbound to SR 167 and SR 167 northbound to I-405.

The freeway turns northeast and runs below several hills with residential neighborhoods overlooking downtown Renton. I-405 intersects SR 515 in a half-diamond interchange and then enters a series of "s-curves" as it travels around the east side of downtown Renton. After crossing the Cedar River upstream from the Renton Public Library, the freeway passes through a public park and intersects SR 169 and SR 900 at two separate interchanges. The southern junction, with SR 169, is a partial cloverleaf interchange at Bronson Way, while the northern junction with SR 900 is a half-diamond that also marks the beginning of a short concurrency with the latter. The concurrent I-405 and SR 900 then pass the Kenworth truck plant and the Renton Landing shopping center near the Boeing Renton Factory before reaching a junction with Sunset Boulevard, which carries SR 900 east towards Issaquah.

From downtown Renton, I-405 narrows to six lanes and climbs over Kennydale Hill and descends down May Creek. The freeway, now closely following the shore of Lake Washington and the Eastside Rail Corridor trail, passes the Virginia Mason Athletic Center, the headquarters of the Seattle Seahawks football team. I-405 leaves Renton and briefly enters the city of Newcastle before continuing into the residential Newport neighborhood in southern Bellevue. The freeway travels along the lake and near several residential areas, passing through two intermediate interchanges at Newport Hills and Coal Creek near Newport High School. I-405 then passes the Marketplace at Factoria shopping center and the headquarters of T-Mobile US before it reaches a full stack interchange with I-90, which provides access to Seattle, Mercer Island, Eastgate, and Issaquah.

The freeway continues north from the interchange towards Downtown Bellevue, passing the Mercer Slough estuary and the historic Wilburton Trestle as it widens to ten lanes. I-405 travels through several interchanges and forms the eastern border of Downtown Bellevue, a major office district, separating it from the retail areas of Wilburton. It intersects Northeast 4th Street in a diamond interchange, Northeast 6th Street in an HOV-only Texas T interchange serving the Bellevue Transit Center and under the 2 Line light rail guideway, and Northeast 8th Street in a cloverleaf interchange near the Overlake Medical Center. A set of four HOT lanes begin at the Northeast 6th Street ramp, traveling north with limited entry and exit points.