|The Old U.S. Highway 91 Historic District is eligible for listing in the National Register of Historic Places under Criteria A and C at a local level of significance. Under Criterion A, the highway follows an historic transportation corridor utilized by humans for thousands of years. The route traverses two scenic canyons which were first documented by the Lewis and Clark Expedition in 1805. Beginning in 1860, the route also included a segment of the first engineered road in Montana and became a significant freight route between the head of navigation on the Missouri River, Fort Benton, and the mining camps and other communities in southwestern and western Montana beginning in 1865. That route later evolved into an automobile highway that was extensively reconstructed in the late 1920s and early 1930s. Today, the transportation corridor includes a 1930s vintage highway, a railroad, and Interstate 15, all of which attest to the corridors significance to the history of transportation in Montana. Under Criterion C, the Old U.S. Highway 91 Historic District contains two large Missouri River bridges, several smaller reinforced concrete bridges, a stone retaining wall, five (of eight) historic culverts, and five intact timber stock passes all which possess considerable integrity and represent bridge engineering practices in the 1930s. Even with the construction of Interstate 15 in the 1960s, the old highway retains most of its original alignment, construction width and other features associated with it during the 1930s. Although the setting of the site has been diminished by the construction of Interstate 15 and a large number of residences built within the last thirty years, the landscape still dominates the corridor and its impact on the design and construction of Old U.S. Highway 91 is clearly evident. The Period of Significance begins at the date U.S. Highway 91 was completed and opened for traffic between Mile Posts 0.0 and 31.6 in 1933 and ends when Interstate 15 was completed and opened for traffic within the same corridor in 1967. After 1967, U.S. Highway 91 functioned as a frontage road for the Interstate.