Obstructions at street intersections. At all street intersections, no obstructions to vision, such as a fence, wall, hedge, structure or planting over three feet in height, as measured above curb level, if any, or above the existing roadway grade, shall be erected or installed and maintained within the triangle 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 30 feet distant from their point of intersection measured along said street lines and/or projections. This subsection shall not, however, be construed to apply to existing street trees, provided that no branches are maintained closer than six feet to the ground.