I don't live in the desert, so maybe I don't get the humor. What does the 37 1/2 sign have to do with anything? Thanks, Kristin :)

Michael Wing

Dear Kristin:
When you're out in the desert, tha landscape is so bleak that you notice things more.  Little things, like a speed limit that isn't a multiple of five miles per hour, seem like big things.
This sign isn't on a state or county-maintained road.  It is at the entrance to a borax mine, so it is on a private road.  There were other random speed limits posted as you approach the gate like "11 miles per hour." 
The signs actually catch your attention much more than signs like "25 miles per hour" would, so it achieves the mine company (Rio Tinto's) purpose which is safety. 
The point is they want you to slow down.  People in the desert always drive too fast because the visibility is excellent and the roads are empty and well surfaced.
The signs are funny because they draw your attention to the absurdity of numerical limits.
Yours, Michael Wing