Tempe police have said that the car didn't brake ("significantly") prior to the collision[1]. So it does not seem that the car reacted optimally but simply had too much speed to halt in time.
I haven't any insight to whether or not the car did or didn't attempt to brake, but it's necessary to respond to the "superhuman reaction speed" remark as, even if the reaction speed is superhuman, that doesn't necessarily mean that it's enough.
Accidents can still occur even if the car is perfect.
[1]http://www.phoenixnewtimes.com/news/cops-uber-self-driving-c...