IS THIS AMERICAN ENGLISH? OR ARE WE ON THE MOON?

Yeah, below, not above.  This is a real sign, not a staged one.  It can be found at an exit from a conference hall where I have teaching sessions at least twice per week. Every time I use the doorway where this sign reigns supreme, I check the ceiling to see if it offers safer passage.  Truth to tell, it is always a lot less congested than the floor level pathway and it doesn't have the obtrusive raised sliding door channel that presents the danger of tripping.  The channel above is recessed and therefore is a whole lot easier to negotiate.  I speak for us humans, not really knowing from first hand experience how the flies think about the situation.  
It should be clear to you by now that I have carefully studied this situation.  After all, I worked for many years providing safety for employees as the Workers Compensation Administrator for large companies.  It was very common practice for us to warn our employees and visitors of all impending dangers, both below and above.  We were so clairvoyant as to warn people of dangers from the right and the left too.  We were thorough.  I must say that in all my years of cautionary sign making, it never occurred to me that we ran the risk of having anyone making a misstep above.  The major reason for that was that our ceilings were always flat and we did not have the above pathways endangered by running extension cords, deteriorated thresholds  and quick, unexpected drop-ons.  We actually had very few objects in the building that could injure the unsuspecting or inattentive employee by flying up and splitting them in half.
In order to make life easier for myself, I have decided to avoid the dangers of a doorway in a building that assures me that the safer way to step would be above rather than below.  
For your sake, I hope that you don't have too many of these in your life.  They sure are weird.