In all districts where permitted, billboards shall be set back from the right-of-way line of any street or highway at least as far as the required front yard depth for a principal building in such districts; however, the setback of any outdoor advertising sign or billboard (not including, however, business identification and directional and other incidental signs otherwise permitted under the provisions of this Zoning Ordinance) on corner lots, in the triangle formed by the lines of streets intersecting at an angle of less than 60 degrees and a line joining points on such lines 100 feet distant from their point of intersection, no outdoor advertising sign or billboard shall be permitted. No such sign or billboard shall be permitted which faces the front or side lot line of any lot in any R District used for residential purposes within 100 feet of such lot lines or which faces any public parkway, public square, or entrance to any public park, public or parochial school, church, cemetery or similar institution, within 300 feet thereof.