Notwithstanding other provisions of this title, and except for routes where a scenic route corridor has been adopted as a part of the specific plan for areas of environmental significance, no advertising sign shall be located or constructed in any district within three hundred (300) feet of any existing right-of-way line of any scenic route as depicted on that map entitled Scenic Route Element of the General Plan, Alameda County, April 1966, said map on file with this commission. (Prior gen. code § 8-60.67)