Interstate 90 in Washington

Interstate 90 (I-90), designated as the American Veterans Memorial Highway, is a transcontinental Interstate Highway that runs from Seattle, Washington, to Boston, Massachusetts. It crosses Washington state from west to east, traveling 298 miles (480 km) from Seattle across the Cascade Mountains and into Eastern Washington, reaching the Idaho state line east of Spokane. I-90 intersects several of the state's other major highways, including I-5 in Seattle, I-82 and U.S. Route 97 (US 97) near Ellensburg, and US 395 and US 2 in Spokane.

I-90 is the only Interstate to cross the state from west to east, and the only one to connect the state's two largest cities, Seattle and Spokane. It incorporates two of the longest floating bridges in the world, the Lacey V. Murrow Memorial Bridge and the Homer M. Hadley Memorial Bridge, which cross Lake Washington from Seattle to Mercer Island. I-90 crosses the Cascades at Snoqualmie Pass, one of the busiest mountain pass highways in the United States, and uses a series of viaducts and structures to navigate the terrain. The freeway travels across suburban bedroom communities in the Seattle metropolitan area, the forests of the Cascade Range, and the high plains of the Columbia Plateau.

The crossing at Snoqualmie Pass was established as a wagon road in 1867 and incorporated into a cross-state auto trail, known as the Sunset Highway, in the early 1910s. The Sunset Highway was incorporated into the national highway system in 1926 as part of US 10, which I-90 replaced when it was designated in 1957. The first segments of the freeway, located in Spokane and Spokane Valley, opened at around the same time and the state government completed upgrades of US 10 to Interstate standards for most of the route by the late 1970s.

The section of I-90 between Seattle and I-405 in Bellevue was delayed for decades because of environmental concerns and lawsuits by local groups over the freeway's potential impact on nearby neighborhoods. A compromise agreement was reached by the federal, state, and local governments in 1976 to build a second floating bridge across Lake Washington and include extensive parks above tunneled sections of I-90, which were completed in the late 1980s and early 1990s. The new floating bridge opened in 1989 and carried bi-directional traffic while the original floating bridge was renovated. The old bridge's center pontoons sank during a November 1990 windstorm due to a contractor error and were rebuilt over the following three years, reopening to traffic on September 12, 1993, marking the completion of the transcontinental highway.

Interstate 90 is the longest freeway in Washington state, at nearly 298 miles (480 km) in length, and is the only Interstate to traverse the state from west to east across the Cascade Mountains. It is listed as part of the National Highway System, classifying important to the national economy, defense, and mobility, and the state's Highway of Statewide Significance program, recognizing its connection to major communities. The Washington State Legislature designated the highway as the "American Veterans Memorial Highway" in 1991 to honor U.S. soldiers. A 100-mile (160 km) section of I-90 between Seattle and Thorp named the Mountains to Sound Greenway was designated in 1998 as a National Scenic Byway, in recognition of its scenic views.

I-90 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. A section of I-90 in Bellevue's Eastgate neighborhood carries a daily average of 150,000 vehicles, making it the highway's busiest. The highway's least busy section, near SR 21 west of Ritzville, carried 11,000 vehicles in 2016. The freeway has a maximum speed limit of 60 miles per hour (97 km/h) in urban areas, 65 mph (105 km/h) in mountainous areas, and 70 mph (110 km/h) in rural areas. Several proposals to raise the speed limit of the rural section between Vantage and Spokane to 75 mph (121 km/h) have been submitted and denied by the state government due to safety concerns.

I-90 begins its eastward journey at the intersection of Edgar Martinez Drive South (part of SR 519) and 4th Avenue South in the SoDo neighborhood south of Downtown Seattle. The interchange is adjacent to T-Mobile Park, home to the Seattle Mariners baseball team, and includes a pair of ramps to SR 519 and an additional offramp to 4th Avenue South north of Royal Brougham Way and near Lumen Field. The ramps converge over the Stadium light rail station adjacent to King County Metro's bus bases and were formerly joined by bus-only express lane ramps from the Downtown Seattle Transit Tunnel and 5th Avenue in the International District. I-90 travels east through a major interchange with I-5 at the northwest corner of Beacon Hill and passes under the Jose Rizal Bridge.

The freeway wraps around the north end of Beacon Hill and intersects Rainier Avenue at the site of the future Judkins Park light rail station, joined by a multi-use bicycle and pedestrian trail that forms part of the Mountains to Sound Greenway. I-90 travels east into the Mount Baker Tunnel, a set of tunnels which run under Sam Smith Park and the Mount Baker Ridge neighborhood to Lake Washington; they are also listed on the National Register of Historic Places. At the east end of the tunnels, traffic continues onto a pair of floating bridges; the eastbound lanes are carried by the Lacey V. Murrow Memorial Bridge, while the westbound lanes, multi-use trail, and future light rail tracks are carried by the wider Homer M. Hadley Memorial Bridge. The two floating bridges connect Seattle to the Eastside suburbs and are among the longest in the world, at 5,811 feet (1,771 m) and 6,603 feet (2,013 m) in length, respectively.