Interstate 15 (I-15) is a major Interstate Highway in the Western United States, running through Southern California and the Intermountain West. I-15 begins near the Mexican border in San Diego County and stretches north to Alberta, Canada, passing through the states of California, Nevada, Arizona, Utah, Idaho, and Montana. The Interstate serves the cities of San Diego, San Bernardino, Las Vegas, Salt Lake City, Idaho Falls, and Great Falls. It also passes close to the urban areas of Los Angeles, Orange, and Riverside counties, California. The stretches of I-15 in Idaho, Utah, and Arizona have been designated as the "Veterans Memorial Highway". The southern end is at a junction with I-8 and State Route 15 (SR 15) in San Diego, and the northern end is at a connection with Alberta Highway 4 at the Sweetgrass–Coutts Border Crossing.

Route 15, consisting of the contiguous segments of State Route 15 (SR 15) and Interstate 15 (I-15), is a major north–south state highway and Interstate Highway in the U.S. state of California, connecting San Bernardino, Riverside, and San Diego Counties. The route consists of the southernmost 289.24 miles (465.49 km) of I-15, which extends north through Nevada, Arizona, Utah, Idaho, and Montana to the Canada–US border. It is a major thoroughfare for traffic between San Diego and the Inland Empire, as well as between Southern California, Las Vegas, Nevada, and the Intermountain West.