I think they ought to be officially dubbed with Heinlein's word, to take its rightful place beside waldo.

I first encountered them in O'hare, in the below-ground transfer tunnel between United's two terminal wings - the one with the neon sculpture and the electronique background music - cool place; check it out if you have the chance. (It even got an article in Smithsonian mag a few years ago.)

They've been springing up all over since then, even in Philly, where they had just finished redoing the terrazzo floors before tearing them up again to install the slidewalks. But that's Philly for ya - they manage their highways like that, too.