No obstructions, such as structures, parking or vegetation, shall be permitted in any district between the heights of 2 1/2 feet and 10 feet above the plane formed by any two existing or proposed intersecting street or alley right-of-way lines and a line joining points on such lines located a minimum of 25 feet from their intersection. In the case of arterial streets intersecting with other arterial streets or railways the corner cutoff distances establishing the triangular vision clearance space shall be increased to 50 feet.