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 the curb level, if any, or above the existing road level, shall be erected or installed and maintained on any lot 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 thirty feet distant from their point of intersection measured along said street lines and/or projections. This section shall not be construed to apply to existing street trees, provided that no branches are maintained closer than six feet to the ground.