No portable sign, parasite sign or other device of such a nature as to obstruct vision may be located within a fifty-foot triangular space at any street intersection; it being understood that such triangular space shall be determined by a diagonal line connecting two points measured fifty feet along each of the projected street curblines, or approximation thereof, equidistant from the projected and intersecting curblines at the street corner; it being further understood that this section shall not include alley intersections.