Christopher Hart, who heads the National Transportation Safety Board (NTSB), believes we may never reach full automation on US roads.

Auto accidents kill more than 33,000 Americans each year, and companies working on self-driving cars, such as Alphabet and Ford, say their technology can slash that number by removing human liabilities. But Christopher Hart, chairman of the National Transportation Safety Board, says that humans can't be fully removed from control. He told MIT Technology Review that autonomous vehicles will indeed be much safer but will still need humans as co-pilots.

MIT: How optimistic are you that self-driving cars will cut into the auto accident death toll?

Christopher Hart: I'm very optimistic. For decades, we've been looking at ways to mitigate injury when you have a crash. We've got seat belts, we've got air bags, we've got more robust [auto body] structures. Right now, we have the opportunity to prevent the crash altogether. And that's going to save tens of thousands of lives.

Autopilot systems can also create new dangers. The NTSB has said that air pilots' overreliance on automation has caused crashes. Do you worry about this phenomenon being a problem for cars, too?

The ideal scenario that I talked about, saving the tens of thousands of lives a year, assumes complete automation with no human engagement whatsoever. I'm not confident that we will ever reach that point. I don't see the ideal of complete automation coming anytime soon. Some people just like to drive. Some people don't trust the automation, so they're going to want to drive. [And] there's no software designer in the world that's ever going to be smart enough to anticipate all the potential circumstances this software is going to encounter.

The challenge is that when you have not-so-complete automation, with still significant human engagement, complacency becomes an issue. That's when lack of skills becomes the issue. So our challenge is: how do we handle what is probably going to be a long-term scenario of still some human engagement in this largely automated system?

Some people say that self-driving cars will have to make ethical decisions, for example, deciding whom to harm when a collision is unavoidable. Is this a genuine problem?

I can give you an example I've seen mentioned in several places. My automated car is confronted by an 80,000-pound truck in my lane. Now the car has to decide whether to run into this truck and kill me, the driver, or to go up on the sidewalk and kill 15 pedestrians. That would [have to] be put into the system. Protect occupants or protect other people? That to me is going to take a federal government response to address. Those kinds of ethical choices will be inevitable. In addition to just ethical choices, what if the system fails? Is the system going to fail in a way that minimises [harm] to the public, other cars, bicyclists? The federal government is going to be involved.

What might that look like?

The Federal Aviation Administration has a scheme whereby if something is more likely than one in a billion to happen, you need a fail-safe. Unless you can show that the wing spar failing - the wing coming off - is less than one in a billion, it's 'likely' to happen. Then you need to have an alternate load path [a fallback structure to bear the plane's weight]. That same process is going to have to occur with cars. I think the government is going to have to say, "You need to show me a less-than-X likelihood of failure, or you need to show me a fail-safe that ensures that this failure won't kill people." I think setting the limit is going to be in the federal government domain, not state government.

- MIT Technology Review