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 91 and 66 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 Hoover Dam, follow the freeway, many of them link distant power stations to the Los Angeles metro area.
The starting point of Interstate 15 was originally planned to be in San Bernardino at an interchange with the San Bernardino Freeway, Interstate 10 (I-10). This was logical as I-15 was following the old alignment of the historic route U.S. 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 created 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 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 123.77 miles.
I-15 just clips the northwestern corner of Arizona with a total length of 29.4 miles (47 km). The stretch is separated from the rest of the state and has one major exit, at Beaver Dam/Littlefield, Arizona. It includes a spectacular section where the road twists between the narrow walls of the Virgin River Gorge.
I-15 continues through Utah for just over 400 miles (640 km). It is the main north-south connection for the state. The highway approximately follows the old alignment of U.S. Highway 91 from St. George to Brigham City. The highway passes through the fast-growing Dixie region, which includes St. George, Cedar City, and eventually most of the major cities and suburbs along the Wasatch Front, including Provo, Orem, Sandy, West Jordan, Salt Lake City, Layton, and Ogden. Around Cove Fort, Interstate 70 begins its journey eastward across the country. The interstate merges with I-80 for about 3 miles (5 km) from South Salt Lake to just west of Downtown Salt Lake City and also merges with Interstate 84 from Ogden to Tremonton. Along nearly its entire length through the state, I-15 winds its way along the western edge of a nearly continuous range of mountains (the Wasatch Range in the northern half of the state). The only exceptions are when it passes through the mountains south of Cedar City and again north of Cove Fort.
I-15 passes through Idaho for nearly 200 miles (320 km). The highway runs through Pocatello, Blackfoot, and Idaho Falls. It also merges with Interstate 86.
Interstate 15 continues onward through nearly 400 miles (640 km) of Montana through the cities of Butte, Helena and Great Falls, intersecting with Interstate 90, Interstate 115 and Interstate 315. At Sweetgrass, I-15 terminates upon crossing the international border into Alberta, Canada; however, I-15 signage is present on Alberta Highway 4 southbound from Lethbridge to the U.S.-Canada Border.