Signs not exceeding four square feet in surface area, which provide directions or instructions to guide persons to facilities intended to serve the public, such as entrances, parking areas, telephones, rest-room and similar facilities, shall be permitted on any lot, provided that no advertising of any kind, other than a corporate logo, appears on the sign. A maximum of four directional signs shall be permitted on lots of two acres or less and a maximum of 10 directional signs shall be permitted on lots of two acres or more. The maximum height of any directional sign shall be three feet above the adjacent ground level. All directional signs shall be located on the same property as the principal use, provided that they shall not be located in any street right-of-way.