Md. Code, Transportation § 22–404.1

(a) A person may not display on a motor vehicle the insignia or emblem of any motor vehicle club or similar organization, or of a fire company, unless he is entitled to use the same under the constitution, bylaws, rules, or regulations of the club or organization.

(b) A person may not display on a motor vehicle any sign that, in the judgment of the Administration, may tend to create a hazard to safety.