Home to over 58,000, Great Falls lies along the Missouri River in Cascade County, Montana. I-315 provides a fast connection to the commercial strip along 10th Avenue South through the city.
Interstate 315 spurs east from I-15 by Great Falls Market Place shopping center to Country Club Boulevard at 6th Street SW. Measuring just 0.824 miles in length per the 2022 Montana Department of Transportation (MDT) Route log, I-315 is only slightly longer than the shortest Interstate highway, unsigned Interstate 878 near John F. Kennedy International Airport (JFK) in New York City.
Signs for Exit 278 along Interstate 15 reference Business Loop I-15, U.S. 89 and MT 200. I-315 is omitted due to the number of overlapped routes. MT 3 is also routed along the freeway, but equally unsigned.
The Great Falls Business Loop for Interstate 15 loops east via U.S. 89 and MT 200 along Country Club Boulevard and 10th Avenue S to 5th Street south and 6th Street north. The I-315 portion passes over both 14th Street SW and a BNSF Railroad. An 11 block long couplet takes the business route north 0.8 miles from U.S. 89/MT 3-200 (10th Avenue S) to 1st Avenue N and the central business district. 1st Avenue west across the Missouri River to Central Avenue returns Business Loop I-15 solo to I-15 at Exit 280.
Montana Highway 3 begins alongside Interstate 315 from I-15. The state highway connects Great Falls with Billings, and it is cosigned with U.S. 89, MT 200, U.S. 87, U.S. 191 and U.S. 12. The only section of MT 3 that is not cosigned with another route is the segment between U.S. 12 at Lavina and Interstate 90 at Exit 450 in Billings. The extended MT 3 was commissioned around 2000; originally it only traveled between Lavina and Billings.
Interstate 315 was one of two routes in the proposed urban Interstate numerology submitted to the American Association of State Highway Officials (AASHO) by the State of Montana Highway Commission on July 31, 1958. I-315 along with I-115 at Butte were both approved by AASHO on November 10, 1958.
A contract to build a 0.6 mile long section of Interstate 315 was completed by September 15, 1967.1 The lone interchange (Exit 0) at 14th Street SW was added in 1995.