An extensive section of this highway (over ), from approximately Stockton, California, to Portland, Oregon, follows the track of the Siskiyou Trail. This trail was based on an older 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 today's I-5 to move between today's Washington state and California. During the second half of the 19th century, mule trains, stagecoaches, and the Central Pacific railroad also followed the route of the Siskiyou Trail. By the early 20th century, pioneering automobile roads were built along the path of the Siskiyou Trail, notably the Pacific Highway. The Pacific Highway ran from British Columbia to San Diego, California, and was the immediate predecessor of much of US 99. The route of US 99 was in turn used as a basis for much of the route of today's I-5.