Roadside retail sales and service establishments, provided that the parcel proposed for development has roadway frontage of at least 50 feet and that no portion of any structure proposed for development will be more than 300 feet, measured along a line parallel to the roadway, from the closest part of a roadside retail sales and service establishment structure that was in existence on February 7, 1979, and that the proposed use will not unduly burden public services, including but not limited to water, sewer, and roads.