Interstate 15 in California

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 Diego, Riverside, and San Bernardino 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.

South of its junction at I-8 in San Diego, the highway becomes SR 15, extending 6.13 miles (9.87 km) to I-5, about 12 miles (19 km) from the Mexico–United States border. This segment was initially signed as a state route instead of an Interstate, but it is being upgraded to Interstate standards so it would become part of I-15 in the future. Including this segment, the entire length of Route 15 is 295.37 miles (475.35 km) in California.

I-15 has portions designated as the Escondido Freeway, Avocado Highway, Temecula Valley Freeway, Corona Freeway, Ontario Freeway, Barstow Freeway, CHP Officer Larry L. Wetterling and San Bernardino County Sheriff's Lieutenant Alfred E. Stewart Memorial Highway, or Mojave Freeway.

Route 15 is defined as follows in section 315 of the California Streets and Highways Code:

Route 15 is from:
(a) Route 5 in San Diego to Route 8.
(b) Route 8 to the Nevada state line near Stateline, Nevada via the vicinity of Temecula, Corona, Ontario, Victorville, and Barstow.

The section of Route 15 defined in subdivision (a) is not considered an Interstate Highway according to the Federal Highway Administration (FHWA)'s route logs.

I-15 and SR 15 are part of the California Freeway and Expressway System, and are part of the National Highway System, a network of highways that are considered essential to the country's economy, defense, and mobility by the Federal Highway Administration. I-15 from SR 76 to SR 91 and SR 58 to SR 127 is eligible to be included in the State Scenic Highway System, but it is not officially designated as a scenic highway by the California Department of Transportation.

SR 15 begins south of I-5 at 32nd Street near Harbor Drive. After this, SR 15 has an interchange with SR 94, which has been cited as not being up to Interstate standards. The route then meets I-805; however, one can only continue in the same direction that they were going at this interchange, since the overall shape of this interchange is an elongated "X". Between the Polk Avenue and Orange Avenue overpasses, the freeway goes under a city park that was built on top of the freeway during construction in 2001. Pedestrian bridges were also built at Monroe Avenue and Landis Street to reduce the effects of the freeway geographically dividing the community. Between I-805 and I-8, SR 15 follows the former alignment of 40th Street, which was its former routing as a city street. It continues seamlessly into the southern terminus of I-15 at I-8; on the northbound conversion to I-15 at I-8, there is no "End SR 15" sign.^{[citation needed]}