Interstate 115 was part of the proposed urban Interstate numerology for Montana submitted to the American Association of State Highway Officials (AASHO) on July 31, 1958. AASHO approved the proposal on November 10, 1958.
Construction contracts to build Interstate 115 ran concurrent with the those for the I-15/90 overlap through Butte. Contracts to build a 2.6 mile long section of roadway, including all of I-115, and five bridge structures ran simultaneously with completion by September 30, 1964. Contracts to construct the western 2.7 miles of I-15/90 were scheduled to December 31, 1963, and the 5.5 mile section to the east through October 15, 1964.1
The eight mile long stretch of I-15/90 was dedicated on September 24, 1964. Work on the four lane, concrete freeway started in 1960 and cost $8 million.2