look down when you cross the street: The Toynbee tiles

"These plaques are, I think, made of plastic which is somehow baked onto the street. Either that or they are painted on with the paint that is used for roads (to make dividing lines). I think they are plastic though because of the fact that it is a negative image that is left (what would suspend the letters in space if it were an ordinary stencil) (although there are fine lines sometime between lines of text). Unfortunately whatever they are made of is delicate, especially around the edges of the letters and tends to break apart after a while"

sigh, all my streets have are just the same un-mysterious manhole covers..