It could have been any other spring afternoon on the Los Angeles highway, with the California sun glinting off of the royal blue Tesla as the Model S crept its way through heavy traffic. There was one piece of the picture, however, jarringly out of place — the bearded man at the wheel of the Tesla who was, it appears, fast asleep.

In a widely-shared video posted to Facebook on Sunday, the driver adopts a posture familiar to anyone who has curled up in a mid-size sedan: eyes squeezed shut, neck bent sharply toward the window, shoulders wedged into the seat. Most travelers slumbering so soundly assume this position as passengers, however. Drivers do so at their imminent peril.

To those in the middle of the California crawl, the scene was as atypical as it was dangerous — but it was not, in the end, tragic. Although the Model S is not properly a self-driving vehicle, as it lacks the autonomy of Google’s steering-wheel-free bubble cars, Tesla automobiles do have a sophisticated autopilot system. The software can guide a Tesla vehicle along a given path, relying on a dozen sensors to change lanes or exit garages. It also, apparently, allows drivers to briefly dream their way through southern California.

But Tesla’s autopilot system it is meant to be a respite from the stop-go headache of traffic jams, not to cart slumbering drivers along the freeway. The car manufacturer condemned the driver’s behavior, releasing a statement to automobile magazine Motor Trend: Tesla’s autopilot system, the company wrote, “does not turn a Tesla into an autonomous vehicle and does not allow the driver to abdicate responsibility. Since the release of Autopilot, we’ve continuously educated customers on the use of the feature, reminding them that they’re responsible for remaining alert and present when using Autopilot and must be prepared to take control at all times.”

In a 2014 Bloomberg interview, Tesla founder Elon Musk compared the Model S’s autopilot to the software that flies airplanes. An expectation of human oversight remains. “We’re not yet at a stage where you can go to sleep and wake up at your destination,” Musk said. That moment — true vehicular autonomy, where you could in fact fall asleep — was about 5 or 6 years away in 2014, in Musk’s view. Add a few more years beyond that, he said, before regulators give driverless cars their approval. (Though some roboticists remain doubtful humans will ever fully cede control of their vehicles to a computer.)

The temptation to fall asleep or become drowsy while on the road is, of course, older than the Tesla Model S or Google’s self-driving car. In the 1960s, researchers proposed the concept of highway hypnosis — that motoring down the freeway was so monotonous or fatiguing it can induce a trance-like state. Fatigue on the interstate is a dangerous state of mind: The National Highway Traffic Safety Administration estimates that, in the four years between 2005 and 2009, more than 80,000 drivers crashed because they were drowsy. If monotony is a problem when a driver still has to steer, some experts are worried that autonomy will exacerbate the issue.

There is a possible solution: If drivers at the wheel of automated cars use smartphones and tablets, they may be less likely to doze off while their vehicles are on autopilot. On its face, this may seem like a paradoxical answer — the Centers for Disease Control, for example, has long cited cellphones and texting as a particularly dangerous habit while driving. But, as David M. Sirkin, an autonomous vehicle expert at Stanford University, wrote in an email to The Washington Post, “it’s possible to forestall the effects of drowsiness induced by automation.” It may be distracted driving, but it’s distraction to keep people alert.

In a small 2015 study, Sirkin and his coauthors had three groups of subjects — representing 48 college students, all between the age of 18 and 24 — monitor a self-driving car during a 40-minute-long simulation. Two groups watched a video or read a passage on a tablet, and the third cohort had nothing else to do but watch the car drive itself. Of the students told only to keep an eye on the computer and the road, 13 showed signs of drowsiness — frequent yawning, for instance, or prolonged periods of closing their eyes — whereas only 3 of those using a tablet seemed as somnolent, the researchers reported. Autonomous car designers, they conclude, may have to walk a fine line between drowsiness and distraction. 

On Tuesday, Tesla announced that drivers had traversed 100 million miles on autopilot.  That’s promising for the future of self-driving cars — but Musk’s company might want to make sure that the drivers remain well-stimulated all the way home.