I have never driven a (partially) self driving car but my experience of regular cars is that I struggle to keep my attention on the road (or staying awake) in long trips as a passenger while I have no such problems as a driver. So I could imagine how the driver of a self driving car, particularly after a long time with no incident, could have low reaction times while given even less time to react than if he was driving.

In any case, self driving cars will have accidents, not the least because some accidents are unavoidable (random behaviour of other cars, animals or pedestrians, unexpected slippery roads, etc), but running heads on into a large visible static object should never happen. It’s a bug.