[devicenull]

I guess you could encode the GPS coordinates of the marker within it, and have the car verify that. Still not a great way of doing it though.

[arethuza]

If you know your location via GPS and connection to the net, why not just look up the instructions relevant to that location?

[mike-cardwell]

And when the key is compromised, have fun replacing every single QR code.

[buro9]

But if the signs are electronic, as in... providing the location of the nearest empty parking space... then updating them should be effortless.

Lots of assumptions here from everyone about how the signs are implemented and what the QR codes contain, but the general gist is that they could be secured. Assuming otherwise is also an assumption.

[Someone]

a) I do not see how having 'electronic signs' would be a good idea. Why bother updating a LCD or so with a QR code if you could just as well use BlueTooth to communicate? The latter likely is more robust and cheaper.

b) The idea is not to have electronic QR signs, it is to have QR signs that contain something like a URL that contains up-to-date info about a place. Replace that URL by your own, and have fun :-)