Idaho Code § 40-2314

Passageways for stock passing beneath any highway must be bridged with suitable plank not less than eighteen (18) feet in length, and it shall be lawful for the fences of either side to converge to the bridge over the passageway. The passageway must be kept securely bridged and in good repair by the person who owns the adjoining lands. The bridge shall not be placed more than one (1) foot above the level of the passageway. Approaches to the bridge over the passageway shall be kept in good repair by the owner.