Maintain trees which are 18 feet or more in height and are within 100 feet of any building or structure or within 10 feet of that portion of any highway, street, alley or driveway which is improved or used for vehicle travel or other vehicular purposes, so that no leafy foliage, twigs, or branches are within 6 feet of the ground. Trees and shrubs less than 18 feet shall be trimmed up 1/3 their height.