Robot cars will inevitably have to be designed to avoid obstacles, because you never know what they might encounter, but that leads directly to robo-griefing. Pranksters will deliberately obstruct robot cars in such a way as to make them much less useful to everyone. I expect a few cases of robot cars being obstructed by small, non-obvious obstacles, like reflectors, or large shapes that look like obstacles, but are no real threat, like cardboard cut-outs. I also expect there will also be cases of bicycles and pedestrians deliberately blocking roads to protest robot cars, but I don't know what their point will be.
Mokalus of Borg
PS - It's hard to claim that something is unsafe when it is refusing to hurt you.
PPS - So that can't be it.