There's no question that machines have faster reaction times, but the other part of the equation is predicting how much a vehicle should slow down in any given situation in order to reduce braking time. The parent was talking about looking at a fairly complicated scene and making a decision about reducing their speed so that they could stop more quickly if it becomes necessary. We all know that there's a big difference between driving by a child playing aimlessly and somebody walking predictably on a sidewalk. It's much more difficult for an automated system to make these sort of predictions than it is for them to react quickly to emergency situations. My guess is that automated driving systems will have to err on the side of caution and that this will result in seemingly (to the passenger) unnecessary slow-downs. I don't personally think that this is a huge problem, but it's fair to say that reaction time is only one one of several significant factors in avoiding accidents.