[Cryptography] How to crypto secure speed limit signs

[Philip Gladstone]

On 3/25/15 09:39, Henry Baker wrote:
> FYI -- In order to keep these Fords from being spoofed, we would need to "secure" each speed limit sign with a crypto signature.  Presumably, this could be done with a QR code.
>
> But how to avoid a "replay" attack -- i.e., cloning an existing sign & installing it somewhere else?  Should the QR code crypto sign the sign's GPS coordinates?  Wouldn't that make speed limit signs pretty expensive to manufacture & install?
You also need to worry about the difference between the actual speed 
limit on the road, and the speed limit according to some path through a 
set of speed limit signs. There was a big expose a few years ago in the 
US where it turned out that towns were putting up artificially low speed 
limit signs in order to get speeding fine revenue. The problem was that 
they were putting these signs on state roads illegally. For example: 
http://boston.cbslocal.com/2012/05/07/i-team-nearly-900-illegal-speeding-tickets-written-in-salem-from-2010-11/

A related question is "what defines the speed limit at any point on a 
road?" Once you can answer this, then "Is there an algorithm which a 
driver (or car) can follow to determine this speed?"

Philip