Interstate 5 (I-5) is the main Interstate Highway on the West Coast of the United States, running largely parallel to the Pacific Ocean and U.S. Highway 101 along the coastline from Mexico to Canada (California to Washington). It serves some of the largest cities on the U.S. West Coast, including Seattle, Portland, Sacramento, Los Angeles, and San Diego, and also links the capital cities of Washington (Olympia), Oregon (Salem), and California (Sacramento) together. The highway's southern terminus is the Mexico – United States border and its northern terminus is the Canada – United States border; it is the only Interstate highway to touch the U.S. borders with both Canada and Mexico. Upon crossing the Mexican border at its southern terminus, Interstate 5 continues to Tijuana, Baja California as Mexico Federal Highway 1. Upon crossing the Canadian border at its northern terminus, it continues to Vancouver, British Columbia as British Columbia Highway 99.
 

Interstate 5 links to control cities in California (San Diego, Santa Ana, Los Angeles, Oakland, San Francisco, Sacramento, Redding, Mount Shasta, Weed, and Yreka), Oregon (Ashland, Medford, Grants Pass, Roseburg, Eugene, Salem, and Portland), and Washington (Tacoma, Seattle, Everett, Mount Vernon, and Bellingham). Vancouver, British Columbia becomes the highway's control city north of Seattle; it is signed as "Vancouver B.C." in order to avoid confusion with Vancouver, Washington.

 

One notable control city that Interstate 5 does not directly reach is San Francisco, which is about 80 miles (130 km) west of the highway. To the south, in the Tracy area, Interstate 580 splits from I-5 towards San Francisco, while to the north, near Vacaville, Interstate 505 cuts south to Interstate 80, which serves that city. That routing, via I-580, I-80, and I-505, was planned originally as Interstate 5W.

 

History

An extensive section of this highway (over 600 miles (970 km)), from approximately Stockton, California, to Portland, Oregon, follows very closely the track of the Siskiyou Trail.[5] This 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 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.[5] 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 U.S. Route 99. The route of U.S. 99 was in turn used as a basis for much of the route of today's I-5.

 

A major deviation from the old U.S. 99 route is the Westside Freeway portion of I-5 in California's Central Valley. To provide a faster and more direct north-south route through the state, the decision was made to build a new freeway to the west and bypass Fresno, Bakersfield, and the rest of population centers in the area instead of upgrading the existing highway (which was re-designated as part of California State Route 99).