That's a great question, and my understanding is that the car simply doesn't ask humans to take over in that case. The "handoff" technique is primarily useful

That's a great question, and my understanding is that the car simply doesn't ask humans to take over in that case. The "handoff" technique is primarily useful in situations where the car's confidence in the situation slowly degrades, not for sudden collision avoidance. An example would be a confusing construction area, or navigating with rocks on the road after a landslide. In these areas where road lines and signs aren't as applicable, the car needs a human driver. The human driver is so people aren't abandoning cars at the start of construction zones, not so they can step in to avoid a sudden collision. Google recently did a study and determined it takes over 5 seconds for a human to take over in a scenario, the car won't be able to expect them to handle anything that is happening quicker than that.