[4] An old freeway section of U.S. Route 101 parallel to Interstate 5 near San Diego International Airport is known as "Pacific Highway," and is now locally maintained.

An extensive section of the Pacific Highway (over 600 miles [970 km]), from approximately Stockton, California to Vancouver, Washington, followed very closely the track of the Siskiyou Trail.

The Siskiyou Trail was based on an ancient network of Native American 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 the future Pacific Highway to move between today's state of Washington and "Alta California".

Hill formed the Pacific Highway Association with himself as president and influential supporters from Alaska to California to serve as officers, including attorney Falcon Joslin of Fairbanks, Alaska, member of parliament Alfred Thompson of Dawson, Yukon Territory, mayor Albert E. Todd of Victoria, British Columbia, Frank M. Fretwell of Seattle, Washington, and attorney Frank Branch Riley of Portland, Oregon.