Corner visibility. On a corner lot, in order to provide
visibility unobstructed at intersections, no sign, fence, wall, tree,
hedge or other vegetation, and no building or other structure more
than three feet above the established street grades, shall be erected,
placed or maintained within the area formed by the intersecting street
lines and a straight line joining said street lines at points which
are 25 feet distant from the point of intersection, measured along
said street lines.