The graffiti tiles tend to be located in the middle of busy streets and highways where they wear away quickly or instead they resurface. Apparently tiles are wrapped in tar paper and placed on a busy street early in the morning, then the pressure of heavy vehicles driving over them for weeks on end pushes it into the road surface. Eventually, the tar paper wears away, exposing similar messages:
TOYNBEE IDEA MOVIE 2001 RESURRECT (sometimes "RAISE") DEAD PLANET JUPITER
The tiles might be made of layers of linoleum and asphalt crack-filling compound, but again no-one is sure. I think that something so cryptic that can cause such controversy is just fabulous, but it does annoy me that I might never find out what they are for!