Idaho Code 36-1508(a) states: No person shall: Shoot from Public Highway. Discharge any firearm from or across a public highway.
Idaho Code 36-202(x) states: "Public highway" means the traveled portion of, and the shoulders on each side of, any road maintained by any governmental entity for public travel, and includes all bridges, culverts, overpasses, fills, and other structures within the limits of the right-of-way of any such road.
Any governmental agency could mean any federal, state or local governmental agency. Maintained can mean paved, graveled, graded, snow removal or any other maintenaince. If the road you are speaking about fits this definition, it would qualify as a public highway.