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
Ohio State Route 51 and US 223 end here as well. The original route of State Route 51 was that of current State Route 49 in western Ohio. The route was generally changed from 51 to 49 until the entire route became 49 in 1935. A new State Route 51 was certified in 1955, with its southern terminus where it is now and its northern terminus at then-State Route 120 (close to where I-280 is now) in Northwood. State Route 51 was extended to State Route 2 in Oregon in 1959. In 1989, State Route 51 replaced the route of U.S. Route 223 though