This weakness is one that I think will plague self driving cars as sign recognition will be key and without some ability to insure that they cannot be dangerously fooled, it will be hard to get them certified. The canonical example is to make a no left turn sign recognize as a no right turn sign and have the car go the wrong way on a one way street.

Clearly there is a marketing opportunity for t-shirts that make you recognize as other things. Who doesn't want to show up in an image search for toasters on Google Images ? :-)

But my current best guess on how this issue will be addressed will be with classifier diversity an voting systems. While that just moves the problem into a harder and harder to synthesize data set (something that not only is adversarial to one classifier but gives the same answer on several), I believe it will get us to the point where we can trust the level of work to defeat them is sufficiently hard to make it a non-threat.