Idaho State Highway 51 Idaho State Highway 51 State Highway 51 (SH-51) is a state highway in southwestern Idaho from Mountain Home south to the Nevada border, where it continues as State Route 225 to Elko. It is the major north–south road in Owyhee County. As currently configured in the state's official milepoint log, SH-51 terminates at the BL-84 intersection with American Legion Blvd., and is not otherwise officially concurrent/overlaid with BL-84 (where the signs at I-84's Exits 90 and 99 say "To SH-51/To SH-67") or the former section of US-20 that follows American Legion Blvd. to I-84/US-20 east at Exit 95. As of
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.