It looks like some Cambridge geeks at Lesley University are providing chalk stations and encouraging passersby (along the stretch of Mass Ave between Porter and Harvard Squares) to write stuff down in a kind of passive public dialog.
And maybe there’s some kind of official encoding thing going on? They have a mapping between numbers and letters, but I didn’t quite pick up on how relevant that is. There’s definitely some key cards that they’ve put up places and made available for downloading.
I walked down the street today and didn’t see any of the chalk booths. Maybe I was on the wrong side of the street or just dumb. It’s not clear.