No fence, wall, sign, hedge or planting which obstructs the sight lines at elevations between two and one-half (2'6") and twelve (12') feet above any roadway shall be placed or permitted to remain on any corner lot within the triangular area formed by the street right-of-way lines, or such lines extended, and a line connecting such right-of-way lines at points twenty-five (25) feet for the intersection of the right-of-way lines.