The human failed to intervene. Worthy of note: Google's cars have no intervening humans. They acknowledge that if you're counting on the human to keep you out of trouble, you're guaranteeing that you'll have trouble when the human is least ready.
I figure he got comfortable like a lot of other normal drivers do, maybe it’s my paranoia but I think many drivers would have reacted poorly in his situation. A lot of people drive in predictable places and then they go traveling and they merge at 50 km/h directly in front of a semi going closer to 90 km/h while I sit on the on ramp thinking but I’m the one who hit my head, why are you like this ? Back in drivers ed my instructor randomly said “ I see a cop car, do you” and then went on a story about how he never got a ticket because he was always scanning. His main point was that a lot of people get comfortable and stop scanning and looking for the seemingly unpredictable. I do agree that the likelihood of the human safety driver being aware enough to intervene is low because they aren’t really engaged in the driving process before that point. They really seem like more of a false sense of security.
I'll wager that at 38mph, you wouldn't hit a raccoon. You wouldn't hit a porcupine. You sure as shit wouldn't hit a deer. A pedestrian? So some pedestrians do dumb shit. Homeless pedestrians in particular. I was coming around a 50mph curve once, at night, in the rain, only to find a black dude (a dark black dude) wearing camouflage fatigues pumping his wheelchair in the middle of the middle of three lanes right at me. He waved. He knew he was doing dumb shit, but he did dumb shit anyway. I didn't hit him, and neither did anyone in front of me. Neither did anyone behind me. Was he lucky? Hells yeah. But in general, people are lucky when it comes to cars. That's because humans have, deep in our understanding of the world, the notion that sometimes shit goes horribly wrong and we have to wing it. Robots don't wing it.