Signs over Public Property. No sign shall be located on publicly owned land or inside street rights-of-way except signs required and erected by permission of an authorized public agency. This restriction shall include, but not be limited to, handbills, posters, advertisements or notices that are fastened, placed, posted, painted or attached in any way upon any curbstone, lamp post, telephone pole, electric light or power pole, hydrant, bridge, tree, rock, sidewalk or street. No projecting sign attached to a building shall project over public property more than four feet and in no case be closer than four feet to curb line or edge of street, whichever is more restrictive. Ground signs must be set back at least 18 inches from any public right-of-way.