The road was first commissioned in 1913 as the multi-national Paciic Highway Auto Trail, which ran from Vancouver, BC to San Diego. It was among the first paved roads in the Pacific Northwest. In 1926, the American Association of State Highway and Transportation Officials (AASHTO) set up a new proposed system of national numbered highways. In 1926, many of these proposed highways were approved by Congress and replaced the auto trail system. The Pacific Highway below halfway between Red Bluffs and San Francisco in California became U.S. Routes 101 and 40. The rest became U.S. Route 99, which split from the Pacific Highway at the intersection of new U.S. Route 40 and continued south to El Centro (it was later extended to Calexico and then truncated to Los Angeles even later).
In Washington, U.S. Route 99 closely followed the original Pacific Highway and the present day Interstate 5. In the 1930s and 1940s, U.S. Route 99 was re-routed and straightened out in many places and even became a freeway in certain areas. Around 1954, several sections of U.S. Route 99 became freeways in Washington, including the newly built Alaskan Way Viaduct in Seattle. Around the same time, the new Interstate 5 was being constructed along the route.
In 1964, Washington and California both went through major state road re-numberings, which meant U.S. Route 99 was becoming non-existant in California. In Washington, all U.S. Highways and Interstates became doubled as state routes with the exact same number, creating State Route 99. Also that year, U.S. Route 99 ceased to exist on paper, but continued to exist in Washington and Oregon until Interstate 5 was completed (it was and still is a State Road in California). Oregon shortened U.S. Route 99 to Ashland and it became Oregon State Routes 99, 99W and 99E. In 1969, U.S. Route 99 was officially removed in Washington, becoming State Route 99 only.
State Route 99 was shortened to Fife in the south and Everett in the north, which are still its ending points today. In the mid 2000's, State Road 99 was removed along part of Tukwila International Boulevard, creating a discontinous gap in the highway. The State plans on making the gap larger over time, which will make State Route 509 the main route in the area.
State Road 99 runs over the Alaskan Way Viaduct, Battery Street Tunnel, and Aurora Bridge (officially called the George Washington Bridge), along the last route used by U.S. Route 99. The older routing of U.S. Route 99 through Seattle followed current State Route 522 (Lake CIty Way), the Freemont Bridge, Westlake Avenue and Fourth Avenue to East Marginal Way instead of Aurora Avenue, which was constructed in 1933 for the purpose of carrying U.S. Route 99. Fourth Avenue was still used until U.S. Route 99 was later rerouted along the Alaskan Way Viaduct. The eastern terminus of U.S. Route 10 was at U.S. Route 99 in Seattle before 1969. If one looks hard enough around Seattle, U.S. Route 99 shields are still visible, such as the Columbia Street on ramp for the viaduct in downtown.
U.S. Routes 99, 10 and 410 were the only three U.S. Highways that existed or were planned to exist within Seattle's city limits.
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