On a corner lot in any district, sight triangles shall be required in addition to the right-of-way, in which no grading, planting or structure shall be erected or maintained no more than three feet in height as measured from the mean elevation of the finished grade five feet away from centerline of the grading, planting or structure. The sight triangle is defined as that area outside the right-of-way which is bounded by the intersecting street lines and the straight line connecting the "sight points," one each located on the two intersection street centerlines the following distance away from the intersecting centerlines: arterial streets at 300 feet; collector streets at 200 feet; and local streets at 90 feet. Where the intersection streets are both arterial, both collectors or one arterial and one collector, two overlapping sight triangles shall be required, formed by connecting the "sight point" noted above with a "sight point" 90 feet on the intersection street.