I am actually slightly surprised that road signs haven't been updated to include something like a "barcode" or similar that would make it very easy for the machine to identify what sign it is. You could "paint" over the entire sign with a variation in the infra-red reflectivity with a QR code, for example.
Then the computer would be better at recognizing a sign than a human.
Then the computer would be better at recognizing a sign than a human.