The first approach, favored by Tesla and by most legacy car companies, is based on the idea that vehicles ought to incrementally add driver-assisting technologies so that safety improves and humans eventually do less of the actual driving over time. Those features include advanced cruise control that can steer in certain environments, and functionalities like the Mercedes-Benz S-Class “traffic jam assist,” which trains the car to automatically follow the pace of traffic ahead at speeds under 37 miles per hour. Eventually, the thinking goes, all those stepwise technologies will yield a fully autonomous vehicle.
The second approach, led by Google, is that driverless cars must be built as fully autonomous from the start. Those who favor this strategy, known as a “Level 4” approach to automation, say it makes the most sense to build a car that can perform all safety-critical driving functions so that the only thing a human has to do is enter the vehicle and tell it where to go. A vehicle of this nature wouldn’t need a steering wheel or brakes. It could start with a button. The passengers could take a nap, read a book, and otherwise disengage from the act of getting from here to there. This is the image of a self-driving car that’s most popular in wider culture, but it’s not the car that most companies are focused on building first.

Although Google is working on developing a Level 4 vehicle, it treats the cars it is now testing on public roads in Mountain View, California, and in Austin, Texas, as Level 3—meaning humans are expected to be available to take control of the car with comfortable transition time. (The numeric scale comes from the National Highway Traffic Safety Administration’s vehicle-automation guidelines.) “Both sides have arguments. I wouldn’t rule out either,” said Gerdes, the Stanford engineer. “There is a fundamental change, though, when you say the human is no longer responsible and the vehicle is responsible.”