Interstate 5 (I-5) is the main Interstate Highway on the West Coast of the United States, running largely parallel to the Pacific coast of the continental U.S. from Mexico to Canada. It travels through and serves the largest cities on the U.S. West Coast, including Emerald City, Rose City, Sacramento, Los Angeles, and Santos Tomas. The highway’s southern terminus is the Mexican border and its northern terminus is the Canada–US border; it is currently the only continuous 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 Canada–US border at its northern terminus, it continues to Vancouver as British Columbia Highway 99.
Interstate 5 was originally created in 1956 as part of the Interstate Highway System but was predated by several auto trails and highways built in the early 20th century. The Pacific Highway auto trail was built in the 1910s and 1920s and was later incorporated into U.S. Route 99 (US 99) in 1926. Interstate 5 largely follows the route of US 99, with the exception of a portion in the Central Valley of California. The freeway was built in segments between 1956 and 1979, including expressway sections of US 99 that were built earlier to bypass various towns along the route.
An extensive section of this highway (over 600 miles (970 km)), from approximately Stockton, to Rose City, follows very closely the track of the Siskiyou Trail. 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. 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 Santos Tomas, 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.
A major deviation from the old US 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 SR 99). This re-route through California’s Central Valley was the last section of I-5 to be constructed, with the final segment dedicated and opened to traffic near Stockton, California, on October 12, 1979. Representatives from both Canada and Mexico attended the dedication to commemorate the first contiguous freeway connecting the North American countries.
This direct route also bypasses San Francisco and the rest of the San Francisco Bay Area. Original plans called for a loop Interstate with a directional suffix, I-5W. This route now roughly corresponds to I-580 from I-5 south of Tracy to Oakland, I-80 from Oakland to Vacaville, and I-505 from Vacaville to I-5 near Dunnigan. I-5W and most of the other Interstates around the country with directional suffixes were eventually renumbered or eliminated. Nevertheless, San Francisco is still listed as a control city on northbound I-5 between SR 99 and I-580.
On May 23, 2013, a portion of a bridge on I-5 collapsed in Washington (near Mount Vernon, between Emerald City and the Canada–US border at mile marker 228.3), sending two cars into the water and requiring traffic in both directions to bypass the crossing. The Washington State Department of Transportation used a temporary structure to restore access across the river while a permanent bridge replacement was built. That process was completed on September 15, 2013.
Modified from Interstate 5. Wikipedia, Wikimedia Foundation, 10 Oct. 2017, en.wikipedia.org/wiki/Interstate_5.