Four years ago, Google's self-driving car team recorded some in-car footage that they've unsurprisingly kept under wraps, until yesterday. That footage is of what test users were doing while sitting in Google's (now Waymo's) self-driving car prototypes: "Napping, putting on makeup and fiddling with their phones as the vehicles traveled up to 56 mph," according to Reuters. In other words, completely unprepared to take over if the autonomous systems became overwhelmed and required "hand-off" to a human driver.

"What we found was pretty scary," [Waymo CEO John] Krafcik said on Monday during a media tour of a Waymo testing facility. "It's hard to take over because [the drivers] have lost contextual awareness."

That led Google's self-driving car team, which has now been spun off as Waymo, to ditch the hand-off practice that most other autonomous car technologists were pursuing. Handing off was seen by many as having two advantages: One, to ease drivers into the notion of the car doing some of the work, and two, as a way to get at least partially autonomous cars on the road while their engineers tried to solve the things an autonomous car couldn't handle. But the handing off concept demonstrates a poor understanding of human behavior.

Humans tend to adapt, which has served us well in terms of survival and evolution. But the ability to adapt where fast-traveling vehicles are concerned has potentially deadly consequences. The first time an intelligent teenager learns to pilot a car at highway speeds, the heightened awareness that comes with new experiences has them paying attention to the road, and driving carefully if their instructor is competent. After the experience is no longer novel and muscle memory has enabled them to operate the car smoothly, their attention may wander. In a semi-autonomous car it will wander even further; remember that the man who was killed in that Tesla crash was watching a Harry Potter movie on a portable DVD player at the time of his death.

Waymo's solution, as they revealed yesterday at a press event, is that autonomous cars must be all or nothing. There can be no handing off.

"Our technology takes care of all of the driving, allowing passengers to stay passengers," the company said in report this month.

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Waymo minivans using their all-or-nothing approach are currently being tested in Phoenix, where residents have been invited to sign up to be riders. They're also conducting private testing at an artificial city they've built at an abandoned Air Force base in California.

Waymo's approach is wise. The traditional human ability to adapt, coupled with modern-day humans' complete intolerance for boredom, means that in an autonomous car they will not be able to keep their eyes on the road and remain ready to take over; there are Facebook updates to type, YouTube videos to watch and Tweets to get outraged over.