High Quality Content by WIKIPEDIA articles! Interstate 15 (I-15) is the tenth longest Interstate Highway, following Interstate 35, and it is the fourth longest north–south Interstate Highway in the United States. I-15 passes through the states of Montana, Idaho, Utah, Arizona, Nevada, and California, and from the Canadian border to San Diego County, close to the Mexican border. This Interstate Highway serves the major cities of Butte, Montana, Salt Lake City, and Las Vegas, Nevada. It also passes close to the urban areas of Helena, Montana, Ogden, Utah, Los Angeles County, California, and Orange County, California.