Interstate 15 Business provides access to downtown Escondido as Centre City Parkway. It follows the former routing of U.S. Route 395.
I-15 Business begins at I-15 exit 28 near the southern city limits of Escondido. It runs parallel to I-15 north through the city. It intersects both the westbound and eastbound portions of County Route S6, which in this area is split into one-way roads (which are in turn separated by another street). This intersection provides access to the end of the Sprinter commuter train line. Shortly thereafter, I-15 Business intersects County Route S14, and then interchanges with State Route 78. The interchange is the last interchange on the 78 before its freeway status ends, and there are only ramps for a westbound entrance or eastbound exit. Continuing north, I-15 Business intersects with El Norte Parkway, a major arterial road, before reconnecting with Interstate 15 at I-15 exit 34 near the northern city limits of Escondido. Though the street itself continues northward well on into Temecula as a frontage road for Interstate 15, its designation as Business 15 ends at this interchange.
Interstate 15 Business in Great Falls, Montana links I-15, which bypasses downtown Great Falls to the west, to the center of Great Falls. As its Business Loop designation implies, I-15 Business terminates at I-15 (concurrent with U.S. Route 89 and Montana Highway 200 through western Great Falls) at each end. The southernmost 0.83 miles (1.34 km) of the route from the interchange with I-15 to Fox Farm Road is designated, but not signed, as Interstate 315. It is the second shortest interstate in the country, the shortest being Interstate 878.
Interstate 15 Business in Shelby, Montana links I-15, which bypasses downtown Shelby to the west, to the center of Shelby. The southern terminus of I-15 Business is at exit 363 on I-15; from there it follows U.S. Route 2 into the center of Shelby, then follows Oilfield Road across the railyard and through the north end of town to its northern terminus at exit 364 on I-15, just south of Shelby Airport.
Interstate 15 Business in Mesquite, Nevada, was also designated as State Route 144. It was removed as of December 31, 2005. It covered Mesquite Boulevard and Sandhill Boulevard in Mesquite. The State Route designation was applied to the portion of Mesquite Boulevard from its southern crossing of Interstate 15 to Sandhill Boulevard, then along Sandhill Boulevard to its northern crossing of Interstate 15. The route was 3.132 miles (5.040 km) long.