At a junction in downtown Detroit, a self-driving car stops beside a pedestrian waiting to cross.

Without touching the car, an electric shuttle operated by startup May Mobility, its safety driver Andrew Dykman confidently waves the man across the road.

“I have a pretty good idea of what the car is going to do at any time. So I’m letting him know that it's waiting for him. I guess it just comes from experience,” he explains.

When a car in Uber’s self-driving testing fleet struck and killed a woman in Arizona last year, images of the safety driver were beamed around the world. The woman, Rafaela Vasquez, appears to be looking down before gasping in shock and grabbing the wheel as the pedestrian, Elaine Herzberg, is hit. A police report concluded that she had been watching The Voice on her phone.

The incident put the spotlight on a mysterious and brand-new line of work. Sitting in the driver’s seat and keeping an eye on a self-driving car in training wasn't anyone's job ten years ago (apart from perhaps a handful of Google engineers). Now, there are hundreds, potentially thousands of them worldwide.

Companies send their partially-developed cars out in public to collect data and to examine how they react to real-life situations. Since none of them are yet fully autonomous, they need a human babysitter.