Yes humans are unpredictable, BUT, in the case of cars, there's only a finite number of things you can do that will affect other drivers. Considering other cars (other environmental things ignored for the moment), the other car can turn left really hard, right really hard, brake really hard or accelerate really hard and all of those within limits. You're following a car, it can brake really hard, but not suddenly be travelling backwards. Solve for all those limited inputs and a human can't do something unpredictable on the microsecond scale you're able to react to.