Keep back!

What is it with these emergency vehicles with signs on the back stating "Keep back 500 ft.?"  By the time I get close enough to read that, I'm way closer than 500', and with people behind me, I won't be able to back up.  And do these folks realize just how far 500 feet is?  That's a city block, almost two football fields!  Can barely see the truck from there.  Why do they need all that separation anyway?  Like 400 feet isn't enough space?