Where forty percent or more of the frontage on the same side of a street between two intersecting streets is developed with buildings that have a front yard greater or lesser in depth than otherwise required, new buildings shall be erected no closer to the street than the average front yard so established by the existing buildings, but may be erected using the average front yard so established. In the B-2, M-1 and M-2 Districts, the front yard need not exceed the specified front yard for the District.