23 CFR § 750.706 - Sign control in zoned and unzoned commercial and industrial areas.
The following requirements apply to signs located in zoned and unzoned commercial and industrial areas within 660 feet of the nearest edge of the right-of-way adjacent to the Interstate and Federal-aid primary highways.
(a) The State by law or regulation shall, in conformity with its agreement with the Secretary, set criteria for size, lighting, and spacing of outdoor advertising signs located in commercial or industrial zoned or unzoned areas, as defined in the agreement, adjacent to Interstate and Federal-aid primary highways. If the agreement between the Secretary and the State includes a grandfather clause, the criteria for size, lighting, and spacing will govern only those signs erected subsequent to the date specified in the agreement. The States may adopt more restrictive criteria than are presently contained in agreements with the Secretary.
(b) Agreement criteria which permit multiple sign structures to be considered as one sign for spacing purposes must limit multiple sign structures to signs which are physically contiguous, or connected by the same structure or cross-bracing, or located not more than 15 feet apart at their nearest point in the case of back-to-back or “V” type signs.
(c) Where the agreement and State law permits control by local zoning authorities, these controls may govern in lieu of the size, lighting, and spacing controls set forth in the agreement, subject to the following:
(1) The local zoning authority's controls must include the regulation of size, of lighting and of spacing of outdoor advertising signs, in all commercial and industrial zones.
(2) The regulations established by local zoning authority may be either more restrictive or less restrictive than the criteria contained in the agreement, unless State law or regulations require equivalent or more restrictive local controls.
(3) If the zoning authority has been delegated, extraterritorial, jurisdiction under State law, and exercises control of outdoor advertising in commercial and industrial zones within this extraterritorial jurisdiction, control by the zoning authority may be accepted in lieu of agreement controls in such areas.
(4) The State shall notify the FHWA in writing of those zoning jurisdictions wherein local control applies. It will not be necessary to furnish a copy of the zoning ordinance. The State shall periodically assure itself that the size, lighting, and spacing control provisions of zoning ordinances accepted under this section are actually being enforced by the local authorities.