I-5 has three auxiliary Interstate Highways within Washington: I-205, an easterly bypass of Portland, Oregon, and Vancouver I-405, bypassing Seattle via the Eastside and I-705, a short spur into Tacoma. It is listed as part of the National Highway System and the state's Highways of Statewide Significance program, recognizing its connection to major communities.

Interstate 5 is the only Interstate to traverse Washington from north to south, serving as the primary highway for the western portion of the state. Construction of I-5 was completed in 1969, and several segments of the highway have been widened or improved in the decades since.

The state's planned toll superhighway in the Seattle area was shelved in favor of a federally-funded freeway under the new Interstate Highway System, under which I-5 was created in 1957. The federal government endorsed the creation of a national expressway system in the 1940s, including several bypasses on US 99 that were built by the state in the early 1950s. The Pacific Highway was built and paved over the next decade, and became the northernmost segment of the national U.S. The state legislature incorporated local roads into the Pacific Highway in 1913, connecting the state's southern and northern borders between Vancouver and Blaine. The freeway follows several historic railroads and wagon trails developed during American settlement of western Washington in the mid-to-late 19th century. I-5 also has three related auxiliary Interstates in the state, I-205, I-405, and I-705, as well as several designated business routes and state routes. Most of the freeway is four lanes in rural areas and six to eight lanes in suburban areas, including a set of high-occupancy vehicle lanes in the latter. The segment in Downtown Seattle is also among the widest freeways in the United States, at 13 lanes, and includes a set of express lanes that reverse direction depending on time of the day. I-5 is the only interstate to traverse the whole state from north to south and is Washington's busiest highway, with an average of 274,000 vehicles traveling on it through Downtown Seattle on a typical day. Within the Seattle metropolitan area, the freeway connects the cities of Tacoma, Seattle, and Everett. It spans 277 miles (446 km) across the state of Washington, from the Oregon state border at Vancouver, through the Puget Sound region, to the Canadian border at Blaine. Interstate 5 ( I-5) is an Interstate Highway on the West Coast of the United States that serves as the region's primary north–south route.

Clark, Cowlitz, Lewis, Thurston, Pierce, King, Snohomish, Skagit, Whatcom

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