I disagree with Lex on this.  He is struggling to develop something that will work in a human environment and manage risks based on human unpredictability.  He's also trying too hard to eliminate all risk.  But the key to autonomous driving is being in an environment where there are no humans and all other cars are autonomous and predictable.   In order to do this, you must accept a certain level of risk, in the beginning maybe higher risk if AI messing up than humans.  Once AI is proven to be equal risk and at no additional cost, self driving cars will be the majority, then you'll have an environment where risks will be dramatically lower.   Also you have to accept that there will always be some risk and not aim for zero risk.