Idaho Code § 40-2321
Any person intending to run water across any public highway must first, under the direction and with the approval of the directors of highways of the county or district, or if the highway be the boundary of two (2) counties or districts, then, under the direction and with the approval of the director of highways of both counties or districts, construct a ditch of sufficient size to carry all the water, and must build a substantial bridge, with easy grades on and off the bridge over the ditch not less than sixteen (16) feet wide. When the quantity of water of any ditch is such that a pipe or culvert will carry the water, the water may be conducted across the highway by means of a pipe or culvert, which must be adapted to the surface of the highway, and the highest point of which shall be at least two (2) feet beneath the surface of the highway, be built of a length not less than sixteen (16) feet, and in a substantial manner permitting uninterrupted travel. All such bridges or culverts shall be of concrete, and all pipes of concrete, steel or other mineral substance. No wooden bridges, pipes or culverts shall be constructed, unless it appears to the satisfaction of the respective commissioners that the cost of the bridge, pipe or culvert would be unreasonably increased by being made of concrete, steel or other mineral substance, and that there is not sufficient travel over the highway to make it necessary for the protection and convenience of public travel that the bridge, pipe or culvert be constructed of those materials. The respective commissioners may in their discretion and by resolution, permit the bridge, pipe or culvert to be constructed of wood or other material, but no bridge, pipe or culvert shall be constructed of wood or any materials other than those specified in this section except upon a resolution of the appropriate commissioners setting forth the reasons and particularly specifying the place of the construction. When a bridge, pipe or culvert shall have been constructed as required, and accepted and approved by the director of highways, it shall become county property and be maintained as other county bridges.
Idaho Code § 40-2321