The XKCD you linked is reasonable on the surface of it, but if you mull over it a bit more, you'll see the three glaring omissions:

1) people care much, much less about "hurting" automated systems, and

2) people feel much less responsibility for accidents & failures if they are separated by a level of indirection of automation

3) people feel much less responsible for mass actions that affect anonymous people far away

Together, those three factors will indeed cause more incidents of people attempting to fool the self-driving cars. Especially if done by displaying malicious images via mass devices, like the light-up info boards, electronic billboards etc.. I don't expect much more actual accidents, tho.