Obstruction to vision at street intersections. At all street intersections in all residence districts, and in all other districts requiring a front setback of 15 feet or more, no obstructions to vision (such as shrubbery, hedge, or other natural growth, a fence or wall) exceeding 30 inches in height above curb level shall be erected or maintained on any lot within the triangle formed by the street lines of such lot and a line drawn between points along such street lines 30 feet distant from their point of intersection (see Sketch B below), nor shall the limbs or foliage on any tree obstruct vision or be permitted to grow nearer to the ground than eight feet where such limbs or foliage overhangs or is over or upon land within the triangular area as shown in Sketch B.