Any vehicle authorized to be equipped with flashing, blinking, or alternating amber warning lights pursuant to subdivision A 1 or 2 of § 46.2-1025 may be equipped with a portable changeable message sign. Such portable changeable message sign may be used without prior authorization from the Department of Transportation, provided that (i) such use complies with all state and federal laws, regulations, and policies regarding the use and operation of portable changeable message signs; (ii) such sign is only activated when the vehicle is being used to perform the duties and functions that qualify such vehicle to be equipped with amber warning lights; and (iii) for those vehicles authorized to use such lights pursuant to subdivision A 1 of § 46.2-1025, such sign is only activated when the vehicle is stationary.