. The viaduct was torn down in 2019 after it was replaced by the
State Route 99 tunnel. An extensive section of this highway (over ), from approximately
Stockton, California to
Vancouver, Washington, follows very closely the track of the
Siskiyou Trail. The Siskiyou Trail was based on an ancient network of
Native American Indian footpaths connecting the
Pacific Northwest with California's
Central Valley. By the 1820s, trappers from the
Hudson's Bay Company were the first non-Native Americans to use the route of U.S. Highway 99 to move between today's Washington state and California. During the second half of the 19th Century, mule train trails, stagecoach roads, and the path of the
Central Pacific railroad (later the
Southern Pacific railroad) also followed the route of the Siskiyou Trail. By the early 20th century, pioneering automobile roads were built along the Siskiyou Trail, including most notably the
Pacific Highway. The Pacific Highway ran from
British Columbia to
San Diego and is the immediate predecessor of much of U.S. Highway 99. The highway was continuous pavement by the mid-1930s.
Decommissioning By 1968, US 99 was completely
decommissioned with the near completion of I-5 in Washington and California, but the highway's phasing out actually began July 1, 1964, when Collier Senate Bill No. 64 passed on September 20, 1963. The bill launched a major program designed to greatly simplify California's increasingly complicated highway numbering system and eliminate concurrent postings. The highways that replaced it are •
SR 111 and SR 86 between the
Mexico–US border and Indio. • I-10, replacing US 60 and US 70 between Indio and Los Angeles as well. •
U.S. Route 101 and
SR 110 in downtown Los Angeles. • I-5 from north of downtown Los Angeles to its modern-day split in
Wheeler Ridge before 99's final decommissioning in 1968. In 1972,
AASHTO gave permission to the
Oregon State Highway Commission to retire US 99W, US 99E and US 99 from the national system. The final segments of US 99 were then decommissioned and re-organized into
OR 99W,
OR 99E and
OR 99.
Successor highways All three states have replaced some portions of US 99 with state highways of the same number: •
Washington: of US 99, from
Fife (in
Pierce County) to
Everett (in
Snohomish County), is now
State Route 99. It is mostly a surface-level highway with the exception of the
SR 99 Tunnel through downtown
Seattle. The tunnel was created to replace the
Alaskan Way Viaduct, which was torn down in 2019. • A 4-mile section of the old US 99 through unincorporated
Hazel Dell and
Salmon Creek, north of
Vancouver, Washington is still known as NE Highway 99. • Other portions of the old US 99 are now designated as
SR 505,
SR 509,
SR 529, and
SR 530 or with names such as "Old Highway 99 S.E." or "Pacific Highway S.W." •
Oregon: Most of former US 99 in Oregon now signed as
Oregon Route 99 (OR 99). The route still provides surface-level access to many southern Oregon towns served by I-5. It also provides access to many towns in the
Willamette Valley. Between
Junction City and
Portland, the highway splits into eastern and western routes known as
OR 99E and
OR 99W, respectively. For significant stretches, OR 99 shares an alignment with I-5. Officially, the highway is signed with both route numbers when this occurs; however, in practice, this is often not the case as the OR 99 designation is dropped in favor of I-5. One notable exception is a stretch of OR 99E that runs between
Albany and
Salem, where OR 99E is cosigned along the highway. •
California: The stretch between Wheeler Ridge and Red Bluff is signed as
State Route 99 which makes it California's second-longest state highway behind
SR 1. However, the newly enacted
Historic U.S. Route 99 extends from
Indio starting from
Interstate 10 in the
Coachella Valley all the way down the
Imperial Valley to
Calexico on the US-Mexico border with
Mexicali,
Baja California,
Mexico. ==Major intersections==