No person shall erect any sign at the intersection of any street in such a manner as to obstruct free and clear vision of such intersection, or at any location where by reason of the position, shape or color it may interfere with, obstruct the view of, or be confused with any authorized traffic sign, signal or device. The placement of all signs shall allow for corner visibility to provide an unobstructed view across the triangle formed by joining points measured 20 feet distant along the property line from the intersection of two streets or fifteen feet along both the street and alley line from the intersection of a street and an alley. Within the triangle there shall be no sight-obstructing or partly obscuring sign or supporting structure between the points 30 inches above grade and 60 inches above grade. Vertical measurement shall be made at the top of the curb on the street or alley adjacent to the nearest side of the triangle or if no curb exists, from the edge of the nearest traveled way.