Interstate 5, or I-5, is an interstate highway along the west coast of the United States. Its odd (as opposed to even) number indicates that it is a north-south highway (though in much of the southern half of California it runs in a northwest-southeast direction). Its south terminus is at the international border between the United States and Mexico a few miles south of San Diego. Its north terminus is at the international border between the United States and Canada near Blaine, Washington.
This highway links the majority of the metropolitan areas in California (San Diego, Los Angeles, and Sacramento); Oregon (Eugene, Salem, and Portland); and Washington (Olympia, Tacoma, and Seattle).
A metropolitan area which is not linked by this highway is San Francisco. San Francisco is about 80 miles (130 km) west of the I-5 alignment. Nevertheless, San Francisco is still 'interstate accessible' via Interstate 80 (I-80), a major east-west interstate that junctions with I-5 in Sacramento.
The western branch of Interstate 5, called Interstate 5W, was replaced by I-505 and I-580, the main spur into the San Francisco Bay Area. What is now I-5 would have been designated Interstate 5E.
Of the existing three-digit Interstate highways, I-105 is the lowest number.
Interstate 5 is the only Interstate highway to touch both the Canadian and Mexican borders; it may be considered part of the Pan-American Highway, a road that runs from Alaska, United States to Chile. It continues into Vancouver, Canada as BC Provincial Highway 99, and crosses from San Diego to adjacent Tijuana, where it becomes Mexico Route 1D.
The highest elevation on I-5 is Siskiyou summit, at 4,310 feet, in Oregon, about three miles north of the California border.
There were plans to build a spur into Salem, Oregon, called I-305, as well as a spur in Portland off of I-405, called I-505, but they were never built. However, a stub of I-505 exists as a long exit ramp to US-30.