Interstate 15 Business is a Business loop of Interstate 15 in Great Falls, Montana. The route 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; only Interstate 878 is shorter.
I-15 Business begins at I-15 exit 278 a short distance west of Great Falls. Here, US 89 and MT 200 separate from I-15 and merge with I-15 Business. Additionally, Montana Highway 3 begins at the interchange and follows the three routes to the east. Just east of the interchange with I-15, BL 15 interchanges with 14th Street. The Interstate Highway-standard conditions come to an end a short distance to the east at an intersection with Fox Farm Road. Here, the 0.83-mile (1.34 km) I-315 (designated along BL 15 from I-15 to Fox Farm Road) comes to an end while BL 15, US 89, MT 3, and MT 200 continue across the Missouri River into downtown Great Falls.
Within downtown, the four routes remain concurrent along 10th Avenue South to the parallel one-way couplet of 5th and 6th Streets. BL 15 northbound separates from US 89/MT 3/MT 200 at 6th Street, following the street northward through the city to the one-way 2nd Avenue North near the city center. BL 15 turns to the west, following 2nd to Park Street, then Park south to Central Avenue, where BL 15 northbound rejoins the routing of BL 15 southbound. Between Central Avenue and 10th Avenue South, BL 15 southbound is routed along 1st Avenue North east to 5th Street South, then 5th south to 10th Avenue.
BL 15 follows Central Avenue to the west, crossing the Missouri River once more as it exits downtown. After several intersections with local streets, BL 15 terminates at I-15 exit 280. Central Avenue, however, continues westward from the exit.