To put it simply, “in a dynamic environment, one has a limited time only to make a motion decision. One has to globally reason about the future evolution of the environment and do so with an appropriate time horizon.”

So, basically, in order to have absolute safety, a car has to literally know everything that is about to happen and has to have enough time to be able to adjust for the movement of everyone and everything else. If it doesn’t, there’s eventually going to be a situation in which there’s no time to react—even for a computer.

“If you could make sure the car won’t break or your [car’s] decisions are 100 percent accurate, even if you have the perfect car that works perfectly, in the real world there are always unknown moving obstacles,” Fraichard told me. “Even if you’re some kind of god, it’s impossible. It’s always possible to find situations where a collision will happen.”

Ref: Driverless Cars Can Never Be Crashproof, Physics Says – Vice