The State Highway Department is hereby authorized and directed to cause to be erected upon the property of or rights-of-way of state and federal highways, at locations most appropriate for carrying out the purposes and intent of this act, signs or markers for each prohibited act enumerated herein, of a size not less than thirty (30) inches square with plainly visible wording to inform users of said highways that the acts enumerated herein do constitute a crime and the maximum penalty for violations, and such additional wording as the State Highway Department deems desirable to assist in carrying out the purposes and intent of this act. Any sign or marker so erected or placed shall be placed at a right angle to the roadbed. The location of signs or markers upon the right-of-way shall in no manner interfere with the signs or markers used to designate route numbers or traffic control markers, signs, signals or devices.