I hadn't, until articles about them started to appear -- like the tiles themselves! -- in connection with a recent documentary:
Toynbee Tiles: A Mysterious Puzzle that Spans the Globe
By Eric Harshbarge
.. these linoleum plaques which have been embedded in the asphalt roadways of major cities throughout the world for over two decades are a true conundrum of the present day. They are written in English (though proper grammar and syntax is often lacking), they are fairly easy to access (if you know where to look), and they have attracted the attention of many puzzle enthusiasts in the age of the all-knowing World Wide Web. However, despite all of this, no one really knows what the hell is going on.
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Trying to uncover a motivation for the tiles’ existence will quickly lead one down a rabbit hole populated with science fiction writers (Clarke, Bradbury), playwrights (David Mamet’s short play 4 A.M. may have something to do with all of this), and radio personalities. The tiles may have begun to spring up as early as 1980, but even today, over thirty years later, little is known for sure.
Probably the most extensive investigation of the tiles takes the form of the new documentary film, Resurrect Dead: The Mystery of the Toynbee Tiles, by Philadelphia-based filmmakers Justin Duerr, Jon Foy, Colin Smith, and Steve Weinik. Duerr is a well known musician and artist of the Philadelphia underground scene, and with that city being the epicenter of Toynbee activity, he is also an expert on the tiles. In 2005 (five years after meeting Duerr) Foy started a film project about the plaques, and six years later the work was finally complete. The research for the film took the documentarians around the United States where they visited about one hundred of the Toynbee sites. They even uncovered a previously unsighted tile placed at a rest stop off Interstate-95.
After hundreds of hours of filming and editing, Colin Smith (one of the producers, writers and co-stars of the movie) thinks at least some of the mystery behind the tiles might be elucidated. “Yes, we definitely think we are closer,” he told me through an email interview, “I’m not going to say that we are ready to close the book, because we are not, but we’ve come up with some pretty huge and very satisfying finds. The mystery persists nonetheless.”
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http://www.wired.com/magazine/2011/02/toynbee-or-not-toynbee-tile-finders-try-to-answer-the-question/