Interstate 5 (I-5) is the main north–south Interstate Highway on the West Coast of the United States, running largely parallel to the Pacific coast of the contiguous U.S. from Mexico to Canada. It travels through the states of California, Oregon, and Washington, serving several large cities on the West Coast, including San Diego, Los Angeles, Sacramento, Portland, and Seattle. It is the only continuous Interstate highway to touch both the Mexican and Canadian borders. Upon crossing the Mexican border at its southern terminus near San Diego, the highway continues to Tijuana, Baja California as Mexican Federal Highway 1 (Fed. 1). Upon crossing the Canadian border at its northern terminus in Blaine, Washington, it continues to Vancouver, British Columbia as British Columbia Highway 99 (BC 99).
I-5 was originally created in 1956 as part of the Interstate Highway System, but it was predated by several auto trails and highways built in the early 20th century. The Pacific Highway was built in the 1910s and 1920s by the states of California, Oregon, and Washington, and was later incorporated into U.S. Route 99 (US 99) in 1926. I-5 largely follows the route of US 99, with the exception of portions south of Los Angeles and in the Central Valley of California. The freeway was built in segments between 1956 and 1978, including expressway sections of US 99 that were built earlier to bypass various towns along the route. US 99 was removed in 1972.