Both highways merged back together in Red Bluff, and US 99 continued along the present-day Interstate 5 corridor to the Oregon border.

An extensive section of this highway (over 600 miles [970 km]), 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.

By the early 20th century, pioneering automobile roads were built along the Siskiyou Trail, including most notably the Pacific Highway.

The bill launched a major program designed to greatly simplify California's increasingly complicated highway numbering system and eliminate concurrent postings.

This section of the highway ran through towns such as Corning, Orland, Willows, Artois, Williams, and Maxwell.

As a result, the older 99 route past Bellingham Bay (Chuckanut Drive) was designated as US 99 Alternate.

[citation needed] Beginning in 1952, the other US Route 99 Alternate began in downtown Bellingham and went due north along the Guide Meridian to Lynden and then to Canada.

[citation needed] Travel on U.S. Route 99 is highlighted in a long poem by Gary Snyder, "Night Highway 99".

[citation needed] Route 99 was planned to be featured in Pixar's Cars 3, as confirmed by Michael Wallis.