The airplane depicted is a Piper Pawnee which is commonly used to tow advertising banners. The airplane is obviously in a dive atitude to pick up the banner with a hook trailing behind the airplane. A glider tow plane or a banner tow airplane landing would not be is such a nose low atitude. The Pawnee depicted is diving, will pitch up and the hook will swing between two poles on the ground and catch a rope strecthed between the two poles. The rope is connected to the banner and the plane will pull the banner off the ground. The sign is probably placed to keep people from driving onto the banner pick-up area.