And punk kids would be covering them over with stickers that machine read as something entirely different. :P Not as easy a thing to crack as it could be. You could cryptographically sign them (sign your signs!) and maybe encode approximate GPS coordinates to prevent them from being moved too far from their original position, but that’s a lot of data to encode in a marker.
That's an over-worried worry. Just like existing road signs which also get tampered with, they would be a redundant source of information on top of the other self driving tech that would reduce instances of a human assistant or driver having to intervene.
Because killing people feels bad, but destroying objects feels like a prank (especially to those who don't understand the consequences). I don't think laws hold back punks from murder; customs do.
