Idaho State Highway 51 February 2015, some map products such as Google Maps still show SH-51's former overlay with American Legion Blvd. SH-51's northern terminus is in the city of Mountain Home in Elmore County. After turning south from Airbase Rd. and the intersection with SH-67, the road crosses the Snake River into Owyhee County and passes through the towns of Bruneau, Grasmere, and Riddle. About from the state border it enters the Duck Valley Indian Reservation. As the road crosses into Nevada it becomes State Route 225. The reservation's only major town, Owyhee is about south of the state border. After another through
U.S. Route 51 in Louisiana neighborhood in Jefferson Parish remained in the state highway system as a disconnected part of Route 33 until the 1955 Louisiana Highway renumbering. The remainder of US 51 carried no state route designation after the renumbering, which eliminated unnecessary concurrencies between U.S. and state routes. Paving projects during the late 1920s and early 1930s led to the early realignment of two portions of US 51 in Tangipahoa Parish. The original route between Independence and Amite City followed what is now known as Old US 51 and remained for a time in the pre-1955 state highway system as State Route 1094.