The QR code is just a hash of the information on the sign to confirm that the system is reading the sign correctly. If someone actually puts up an incorrect sig

The QR code is just a hash of the information on the sign to confirm that the system is reading the sign correctly. If someone actually puts up an incorrect sign, it would be immediately apparent to humans. If you really wanted a secure system you'd just have a database of all the signs in an area that cars could access (for example if I know what road i'm on, and my approximate location on that road, I should be able to look up what the speed limit on this stretch of road is). The fact is if you want to be able to stop a car from going over a bridge that's collapsed by placing a sign in front of it, inevitably the same system can prevent a car from going over a bridge that hasn't collapsed. Realistically, unexpected road signs should prompt manual operation.