Attachment of portable signs

(a) A portable sign constructed in accordance with Section 14-280-1 of these regulations shall be mounted securely on or above the roof of the motor vehicle perpendicular to the sides of the vehicle and when in use extend vertically upward from a horizontal plane defining the vehicle roof in a manner that permits the words on the portable sign to be clearly visible to the operators of other vehicles at a distance of between fifteen (15) feet and two hundred (200) feet on a straight line to the front and rear of the motor vehicle. If necessary to provide the required visibility, a second portable sign may be mounted on the roof of the vehicle, or on the rear of the vehicle as permitted in Section 14-280-3 (a). (b) Any mounting brackets, straps, clamps or mounting hardware used to secure a portable sign to a vehicle shall not interfere with normal window or door operation of the vehicle, shall be of sufficient strength to withstand the load applied thereto by the sign, and shall not pose a danger to or interfere with the occupants of the vehicle. Suction cups, magnets or other easily detached devices shall not be used unless in addition to secure mounting hardware. A portable sign may be screwed, bolted or otherwise fixedly secured to a vehicle in a manner that will permit its removal. A layer of protective or non-slip material may be used between a portable sign and/or its mounting hardware and the surface of a motor vehicle.