Since I-15's inception, it has served as a long-haul route for North American commerce. It is now officially chartered for this purpose. From the junction of Interstate 515 in Las Vegas to the Canadian border I-15 forms part of the CANAMEX Corridor, a High Priority Corridor, as a result of the North American Free Trade Agreement.
Local portions were built to connect the Inland Empire with San Diego in California, facilitate tourism access to Las Vegas, interconnect all of the Metropolitan Statistical Areas in Utah except Logan, and provide freeway bypasses for Pocatello, Idaho Falls and Great Falls.
Since I-15's initial construction, California, Nevada, and Utah have consistently ranked in the fastest growing areas of the United States. As a result, the I-15 corridor has substantially increased in population and commuter traffic has increased the traffic burden on the freeway. Current population estimates are that more than 75 percent of Utah's population, 19 percent of California's population, and more than 70 percent of Nevada's population live in counties where Interstate 15 is the primary transportation corridor. Similarly, in California, I-15 is seeing more commuter traffic due to the growth of the Mojave Desert communities of Victorville and Barstow, California. In all of these states, I-15 has recently been or is currently in the process of being upgraded to increase capacity. The Arizona, Idaho and Montana portions have retained their rural, long-haul character. Although Arizona has also grown substantially since I-15's inception, I-15 only serves an isolated corner of the state.
Due to this rapid area growth, the I-15 corridor is the focus of several mass transit projects. The Las Vegas Monorail, FrontRunner commuter rail system and TRAX light rail in Salt Lake City are mass transit lines loosely parallel to I-15 that are now in operation. Between Los Angeles and Las Vegas has long been proposed as a maglev train route; in 2004 the California-Nevada Interstate Maglev project held public meetings on the plan.
Bolded cities are officially-designated control cities for signs
This highway's southern terminus is in San Diego, California at Interstate 8, 18 miles (29 km) north of the international border with Mexico. However, work is being done to extend this to Interstate 5. The current work on this extension is signed State Route 15. There is a continuous freeway between I-8 and I-5, but various intersections and acceleration/deceleration lanes are not up to interstate standards. The northern terminus is in Sweetgrass, Montana at the international border between the United States and Canada, where it becomes Alberta Highway 4.
North of its junction with the Riverside Freeway, State Route 91, in the Inland Empire near Corona, the route roughly follows the former routes of U.S. Highway 91 and U.S. Highway 395. North of Devore, the highway follows the approximate alignment of historic U.S. Highway 66 along with U.S. 91 and 395. U.S. 395 breaks away at Hesperia and the route continues the approximate route of co-signed 66 and 91 until about the Mojave River, 35 miles (56 km) to the north. At that point, I-15 follows the old route of U.S. 91 exclusively. For many parts of the highway, high-voltage power lines, like Path 46 and Path 27, almost all from the Hoover Dam, follow the freeway. Many of these link distant power stations to the Los Angeles metropolitan area.
The starting point of Interstate 15 was originally planned to be in San Bernardino at an interchange with the San Bernardino Freeway, I-10. This was logical as I-15 was following the old alignment of the historic Route 66 which passed through San Bernardino. The segment was completed accordingly. However, legislation was later passed to extend the interstate to San Diego. But instead of extending the existing freeway from the I-10 interchange south, the California Department of Transportation made a new segment in Devore that 'branched' off of the original alignment and bypassed San Bernardino altogether. This segment's alignment is generally northeast to southwest for about 15 miles (24 km). Then, in Fontana/Rancho Cucamonga, its directional alignment shifts to north-south where it eventually junctions with Interstate 10 (about 15 miles or 24 kilometers west of the original interchange in San Bernardino). The segment that had been built from Devore to San Bernardino was retained as an interstate, but was re-numbered as Interstate 215. Note that during the construction of I-15's present alignment, and for some time afterwards, I-215 was numbered as I-15E, and its actual mileage would begin at Interstate 10.
Interstate 15 begins in Primm, continues through Las Vegas along the Las Vegas Strip corridor. Then the interstate crosses the border with Arizona in Mesquite. The whole interstate in Nevada runs entirely in Clark County, for a distance of .