On a corner lot, to provide unobstructed visibility at intersections, no sign, fence, wall, tree, hedge or other vegetation, and no building or other structure between three feet and eight 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 20 feet from the point of intersection of ways or tangents of curves of rounded curbs, measured along said street lines.