Maryland Code, Natural Resources § 5-407

(a) The State Highway Administration may grant to any person the right to place any advertisement, sign, notice, or other writing along or upon the public highways of the State to be used only in conjunction with direction or danger signals, and subject to the limitations and restrictions imposed at the time the permit is granted. A permit may not be issued unless the need for the direction or danger signal to be erected is clearly demonstrated to the satisfaction of the State Highway Administration. Any person doing an act otherwise prohibited in this section, by virtue of a permit issued by the State Highway Administration, is immune from prosecution.

(b) In Kent and Queen Anne's counties, the respective county boards of education may exhibit or display any advertisement, sign, notice, writing, or other device for the purpose of promulgating the safety of students, on or abutting any road or highway which passes near any school. However, these signs may not be displayed farther than 300 yards from any school and any designation of the donor may not comprise a space of more than 2 square feet on the sign.

(c) A person, without first obtaining the written consent of the owner, may not paint, put, or fix any advertisement, sign, notice, or other writing, other than a notice posted pursuant to law, on or to any stone, tree, fence, stump, pole, building, or other structure which is in or upon either the public highway or property of another, or procure, direct, or induce the painting, fixing, or placing of the advertisement or sign.

(d) For the purpose of enforcing this section, the presence of any advertisement, sign, notice, or other writing, other than a notice posted pursuant to law, upon the public highway or private property adjacent to it constitutes prima facie evidence that it was painted, placed, fixed, or erected at the direction of, or with the consent and approval of, the party or his agent or representative in the State whose name, business, location, or merchandise is advertised thereon.