I think that by the letter of the law, liability would be strictly with the "safety driver" (aka scapegoat on duty). They were surely told to keep attentive all

I think that by the letter of the law, liability would be strictly with the "safety driver" (aka scapegoat on duty). They were surely told to keep attentive all the time and - surprise - it did not happen. The root cause, in my opinion, is allowing cars on the road that inevitably lull their nominal driver into inattentiveness. There might be ways to keep a safety driver engaged so that the chance of attention failure is significantly lower than inevitable, but it doesn't seem like Uber had been looking for them very hard.