Corner clearances shall be maintained within that area bounded by the lines of the intersecting streets and a diagonal joining the points on each street line distant 20 feet from the point of intersection, or in the case of a rounded corner, at a point of tangency. Within this area, no structure, wall, embankment, terrace, porch, piazza, fence, hedge, sign, vegetation or other obstruction other than shade trees trimmed to a distance of at least ten feet above the curb line, shall be permitted above a height of two feet above the horizontal plane through the curb line.