Unless a greater setback requirement is prescribed in the Table of Dimensional Regulations for Residence and Agricultural Districts, Row 5, and the Table of Dimensional Regulations for Business and Industrial Districts, Row 6, front yards abutting all county and state highways, Lake Osiris Road, Berea Road, Barron Road, Neelytown Road, Goodwill Road, Chandler Lane, Kings Hill Road, Stone Castle Road, North Drury Lane, and Beaver Dam Road shall be at least 60 feet in depth, except where the majority of existing primary structures on either side of the road within 300 feet from the intersection of the nearest property line and street line is of a lesser average depth. In such cases, the front yard depth shall be 50 feet or the average of all lot depths within said 300 feet, whichever is greater.