Signs

Have you noticed how many strange signs are posted everywhere. When driving to the airport in New York I noticed a sign on the highway "Low Flying Planes". What exactly is that sign's purpose? How low are these planes flying? 
 I was walking in my neighborhood this week and I noticed a sign on the front lawn of a home,
 " beware of invisible fence". Really? Is there such a thing? 
                             And finally a sign from a local pharmacy. Soak up the savings!