All streets, alleys, and railroad rights-of-way, if not otherwise specifically designated, shall be deemed to be in the same zone as the property immediately abutting upon such streets, alleys, or railroad rights-of-way. Where the centerline of a street or alley serves as a district boundary, the zoning of such street or alley, unless otherwise specifically designated, shall be deemed to be the same as that of the abutting property up to such centerline.