[Cryptography] How to crypto secure speed limit signs

[John Levine]

>This won't work in the UK because you need context to interpret the
>'national speed limit' sign which is a black diagonal line on a white
>background.
>
>It could mean 60mph or 70mph depending on the road type.

Assuming the people managing the signs weren't completely dim (which I
realize we can't take for granted), they would presumably program in
the relevant speed along with the GPS coordinates when they install
the sign.

Of course, they could get it wrong which would lead to much jolly
excitement.

R's,
John