The route consists of the southernmost 289.24 miles (465.49 km)[2] 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.

Pedestrian bridges were also built at Monroe Avenue and Landis Street to reduce the effects of the freeway geographically dividing the community.

After traversing the Marine Corps Air Station Miramar, I-15 comes into Rancho Peñasquitos, where it meets the eastern end of SR 56.

Northward, the route crosses Lake Hodges inside the upper San Diego city limits.

North of Escondido, I-15 goes through hilly terrain and farmland, passes under the Lilac Road Bridge and approaches the community of Fallbrook near the SR 76 interchange.

In Murrieta, I-15 splits from its only auxiliary route in California, I-215, which retains the Escondido Freeway designation and runs through the two largest cities in the Inland Empire, Riverside and San Bernardino.

[12] I-15 runs along the eastern edge of the Santa Ana Mountains, passing through the cities of Wildomar and Lake Elsinore.

In Lake Elsinore, I-15 intersects SR 74, a major highway connecting the city with San Juan Capistrano as well as points east such as Perris, Hemet, Idyllwild, and the Coachella Valley.

At this junction, I-15 takes a strongly northeastern alignment as it moves to rejoin with its spur route, I-215, in Devore, in northern San Bernardino.

[13] On this stretch of highway, I-15 northbound splits from I-15 southbound, where the road ascends up a steep grade until it reaches Cajon Summit (elevation 4,260 feet (1,300 m)) just south of the High Desert communities of Hesperia and Oak Hills.

Tractor-trailer trucks headed southbound are required to travel at the posted speed limit of 45 mph (72 km/h) or less due to the steep downward grade.

North of the Cajon Pass, I-15 traverses the High Desert cities of Hesperia, where it meets the southern end of US 395, and Victorville.

The sign for Zzyzx Road—alphabetically the last place name in the world—is a landmark of sorts on the drive between Los Angeles and Las Vegas.

A runaway truck ramp is provided for northbound I-15 traffic near the dry Ivanpah Lake at the end of Mountain Pass.

The Mojave Freeway is fairly busy on weekdays, since it connects the rapidly growing exurbs of the Victor Valley with the Los Angeles area.

[15] For the segment in San Diego County, each solo driver is required to carry a FasTrak transponder, while carpools, motorcycles, and clean air vehicles are not charged.

I-15 was initially planned to run from I-10 near San Bernardino along the current I-215 alignment then up through the Cajon Pass and on to Las Vegas, with a distance of 186.24 miles (299.72 km) within the state.

[21] Meanwhile, the segment of Route 15 from the San Diego County Line to the I-15/I-215 interchange was named the Temecula Valley Freeway in 1990.

[23] This legal definition has been amended twice: First, in 1990, the California Assembly passed Concurrent Resolution 125, Chapter 78, renaming I-15 between the San Diego County Line (which is further south from I-215) and Bundy Canyon Road near Lake Elsinore as the Temecula Valley Freeway.

[22] Then, in 1993, the California Assembly passed legislation officially designating SR 71 as a part of the Chino Valley Freeway.

Because families did not want to live in these houses since they would be soon torn down, they rented them to individuals who were only going to be in the area on a temporary basis, many of whom were involved in illegal activities.

Even though the freeway was officially added to the Caltrans proposals as early as 1968, it was not until March 1992 that construction began after they succeeded to evict residents to make way for it.

[28] Projects include adding 39 miles (63 km) of truck lanes on hills at various locations, repaving 76 miles (122 km) of I-15 at various locations, adding exit numbers, renovating and rehabilitating the rest area between Baker and the Nevada State Line (Valley Wells Rest Area), reconstructing bridges in Baker, and moving the agriculture inspection station from Yermo to the Nevada State Line and including a truck weigh station.

(A 1986 map shows state maintenance continuing north past SR 60 to Jurupa Street, where it turned east to I-15.

Business routes of Interstate 15 exist in Escondido, Lake Elsinore, Norco, Victorville, Barstow, and Baker.