Maryland Code, TRANSPORTATION 8-807
(1) Screened by natural objects, plantings, fences, or other appropriate means so as not to be visible from the main traveled way of the highway;
(2) Otherwise not visible from the main traveled way of the highway;
(3) Located in an area that is zoned for industrial use under authority of local law; or
(4) Located in an area that is not zoned for industrial use, but that is used for industrial activities as determined by the Administration with the approval of the United States Secretary of Transportation under the federal Highway Beautification Act of 1965.
(b) This section does not permit the Administration to preempt any zoning act or ordinance of any political subdivision. If a political subdivision later zones any area for a use that conflicts with a determination of the Administration, the determination of the Administration is void.