Signs indicating the location and direction of premises available for or in process of development, but not erected upon such premises, and having inscribed thereon the name of the owner, developer, builder or agent, may be erected and maintained, provided:
(1) The size of any sign is not in excess of six square feet, and not in excess of four feet in length.
(2) Not more than one such sign is erected on each 500 feet of street frontage.