Boing Boing links the clever short film [ airport ], made entirely from the common airport direction and instruction symbols.

Also cool is the AIGA page where the design association makes all the symbols available in EPS and GIF formats:

This system of 50 symbol signs was designed for use at the crossroads of modern life: in airports and other transportation hubs and at large international events. Produced through a collaboration between the AIGA and the U.S. Department of Transportation, they are an example of how public-minded designers can address a universal communication need.
...
These copyright-free symbols have become the standard for off-the-shelf symbols in the catalogues of U.S. sign companies.

Indeed, I'd suggest that the symbols' freedom from trademark and copyright claims has directly spurred their widespread adoption, which in turn has helped to make them more universally understood. Score one more for the commons.