While there are situations where human cognition is superior to a computers (such as being able to improvise in a new situation), most of the mistakes humans make while driving are due to the weaknesses of being human.

We get tired, we get distracted, we get emotional, we get drunk, we get old, we lose out eyesight, and sometimes we just make stupid, impulsive decisions.

If you look at the most common reasons for car accidents, you'll see this... speeding, drunk driving, running red lights, etc.

Technology to get cars to reliably follow traffic signals, road signs, adjust to road conditions, etc is already largely there though they're really working on putting alot of miles on these vehicles so they can discover as many edge cases as possible before they go mainstream (the biggest risk is other human drivers right now).

I think they can get things to a point where the accidents will either be human error (in terms of the pedestrian or other driver that's hit) or due to a defect in hardware (which can be estimated and managed).