No sign structure may block a vehicular line of sight for a driveway, access lane, or public street, or be placed at any location where by its position, shape, or color it may interfere with or obstruct the view of or be confused with any authorized traffic sign, signal or device. Sign structures erected directly upon the ground within 15 feet of any vehicular driveway, or street intersection shall have at least three feet six inches of clear space between such sign and the ground; however, necessary supports may extend through such open space.