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.

It also passes close to the urban areas of Los Angeles, Orange, and Riverside counties, California.

The stretches of I-15 in Idaho,[citation needed] Utah, and Arizona have been designated as the "Veterans Memorial Highway".

I-15 was built to connect the Inland Empire with San Diego in California, facilitate tourism access to Las Vegas, provide access to the Arizona Strip, interconnect all of the metropolitan statistical areas in Utah except for Logan, and provide freeway bypasses for Pocatello, Idaho Falls, and Great Falls.

[4] Since the construction of I-15, California, Idaho, Nevada, and Utah have consistently ranked in the fastest-growing areas of the United States.

US 395 breaks away at Hesperia and the route continues on a direct path to Barstow 35 miles (56 km) to the north.

Instead of extending the existing freeway from the I-10 interchange south, however, the California Department of Transportation drew a new segment in Devore that "branched" off of the original alignment and bypassed San Bernardino altogether.

Then, in Rancho Cucamonga, its directional alignment shifts to north–south where it eventually meets with I-10 (about 15 miles (24 km) west of the original interchange in San Bernardino).

Note that during the construction of I-15's present alignment, and for some time afterward, I-215 was numbered as I-15E, and its actual mileage would begin at I-10.

It includes a spectacular section where the road twists between the narrow walls of the Virgin River Gorge.

The only exceptions are north of Cove Fort and when it passes between Cedar City and St. George, known as the Black Ridge, a transition zone of drastic change in elevation and climate, an area where the eastern Great Basin, Colorado Plateau, and Mojave Desert converge.

I-15 continues onward through 396 miles (637 km) of Montana through the cities of Butte, Helena, and Great Falls, intersecting with I-90, I-115, and I-315.

Once I-15 was relatively intact, US 91 was decommissioned, except for one part in Northern Utah / Southern Idaho where I-15 instead followed the route of former US 191.

[8] Since the construction of I-15, California, Nevada, and Utah have consistently ranked in the fastest-growing areas of the United States.

Similarly, in California, I-15 is seeing more commuter traffic due to the growth of the Mojave Desert communities of Hesperia, Victorville, and Barstow.

[citation needed] The Los Angeles–Las Vegas corridor has long been proposed as a high-speed maglev train route to relieve highway congestion.