[3][5] A set of business routes, which use a modified green Interstate shield, are designated and maintained by local governments instead of WSDOT and do not require approval from the FHWA.

[14] Early proposals for a national system of "superhighways" with limited access and grade separation emerged in the 1930s at the behest of the Bureau of Public Roads (BPR; now the FHWA) under the Roosevelt administration.

[15][16] The first major expressway with limited access to be built in the state was the Alaskan Way Viaduct in Downtown Seattle, which was announced in 1947 and used federal grants authorized by the Federal-Aid Highway Act of 1944.

[20][21] A second bill in 1951 authorized the construction of expressways to replace rural sections of US 99 and US 10 (the east–west trunk), particularly by bypassing small cities, and the program was expanded to cover 2,388 miles (3,843 km) by 1953.

[28][29] Washington's third primary route, connecting Ellensburg to Pendleton, Oregon, was authorized by the BPR in October 1957 as part of an addition to the Interstate Highway System.

[33] Among the first Interstate projects in Washington to be built with funding from the Federal-Aid Highway Act of 1956 were bypasses of Fort Lewis and Olympia that had already been planned by the state government.

[37] The state government made improvements to the expanding Interstate system in the 1960s and 1970s, building rest areas and scenic overlooks on the primary routes and introducing mileage-based exit numbers in 1973.

[38][39] The southern section of I-82 between Yakima and Oregon was originally routed away from the Tri-Cities region, where local leaders petitioned for access to the Interstate system.

[40] The new connector, numbered I-182, was approved in 1969 and fully opened in 1986 alongside the last Washington section of I-82; the highways were delayed by disagreements with local governments and spending cuts during the early 1980s recession.

[46][47] The final section of the Interstate system in Washington, I-90 between Seattle and Bellevue, took over 30 years to plan and construct amid disagreements and litigation over its design and other mitigation.

[55] A 2004 study commissioned by the state legislature determined that an outer beltway would not be feasible to construct due to its high costs as well as opposition from local residents and environmental groups.