(14) Department rules. The department may promulgate rules deemed necessary to implement and enforce this section. The department shall promulgate rules to restrict the erection and maintenance of signs as to their lighting, size, number and spacing when such signs are visible from the highway but outside the adjacent area. The department shall by rule establish a priority system for the removal or relocation of all signs not specified in sub. (5) (d) which fail to conform to the requirements of sub. (5).