Obstructions at street intersections. On a corner lot in any residence district, no fence, wall, hedge, structure or planting over 2 1/2 feet in height or closer than five feet to the ground shall be erected, placed or maintained within the triangular area formed by the intersecting street lines or their projections, where corners are rounded, and a straight line joining said street lines at points which are 50 feet distant from the point of intersection, measured along said street lines and/or projections.