While an AI might specifically already know that a given intersection is supposed to be controlled, there are large numbers of driving situations in which memory (or maps) aren't very useful in dealing with.
When it comes to self-driving cars, probably the way around attacks on signs or other static road features is just to fail safe even in situations in which you're mis-directed by road features (that is, you drive cautiously enough that even if you don't understand the road situation, you can find a way to stop without hurting anyone). But that's not a general AI solution, and may not even be a general self-driving car solution.
Deep neural nets should really be called "really complicated composition of differentiable functions" and all the training algorithms should really be called "really complicated function root finding" because that's all these things are at the end of the day, a rats nest of functions with a billion knobs.
That's not the reason at all.
It's because if you add exactly the right type of specially crafted noise to an image of a stop sign such that, to a human, it looks like a panda--then who is going to be the arbiter that decides "No, it's still actually really an image of a stop sign and not a panda". The machine?
The real reason is the subjective nature of the perception of reality. With the advent of AI, all sorts of schools of philosophy that used to seem like useless navel-gazing are going to find some real important practical applications real soon. This is metaphysics and ontology. On a similar note (not in TFA), in the "friendly AI" debate, we're going to be looking real hard at the foundations of the philosophy of ethics (where does it really come from?), not just the teleological vs deontological (etc) debates we use to reason about law, politics, governance and appeasement of our "justice" instincts.
Or quite possibly we just don't know how to construct the noise yet :P
Isn't that illegal in most jurisdictions?
Eg. look at the panda images at the start of the linked post. The third image looks like a panda to humans (and most humans probably wouldn't be able to see the difference between the first and third images), but it greatly affects the machine learning interpretation.
By modifying the sign in this way, if it is possible, it would be much harder to detect and enforce against than it would be if the attacker was targeting human drivers.
An image that triggers a false positive within the AI won't be noticeable to the average human, however good at detecting bullshit as they are. It probably looks like some annoying abstract pattern at the worst.
To the car, however, it could represent anything that made it react adversely upon observing it. Seems like figuring out how to find those images might pose a challenge.
In the case of a human not seeing a stop sign, some drivers will remember a stop sign used to be at that location, but a self-driving car is less likely to forget and might (in a world with networked cars, which I find slightly scary for different reasons than this) be able to "remember" that other cars had previously seen a stop sign at that location.