David, this is indeed interesting. I’m currently working my way through the Udacity SDC nanodegree and have been contemplating this quite a lot especially since last weeks news on researchers being able to hack the signs if they know the AI algorithm that does detection. I keep coming around to a blockchain style solution that would allow vehicles to authenticate legitimate signs. I think though, like most security solutions, there will be layers and multiple factors to authenticate a sign — things like GPS based varification, is the sign in a database, was the sign reported there by the last x number of vehicles over the last y numbers of days/weeks/months. Is this a known construction zone, etc. Is this the only sign that has changed recently? Does this sign make sense on a surface street vs. a highway, etc. Ground truth is going to be a sub-industry all of it’s own for sure. Good stuff.