From the desert to the city
The modern field began as a test. In the 2004 DARPA Grand Challenge no vehicle finished the desert course; a year later five did, and in the 2007 Urban Challenge robots negotiated a mock city with traffic and intersections. Those events proved the basic problem was tractable and scattered the winning teams into what became Google's project, Waymo, Aurora and much of the industry. They also set a pattern: prove it on a course before the public road.
Simulation at scale
Public roads cannot supply enough of the rare events that matter, so most testing happens in software. Companies replay logged drives with the new software in the loop, mutate real scenes into harder variants, and generate wholly synthetic scenarios: a child runs out between parked cars, a truck runs a red light. Waymo has reported tens of billions of simulated miles against tens of millions of real ones. A change to the driving software is run through this battery before it reaches a car.
Closed courses and shadow mode
Between simulation and the street sit private test tracks with movable buildings and dummy pedestrians, where the scenarios that would be dangerous in public can be staged. On the road, new software often runs first in "shadow mode": it computes what it would do while the current software, or a human, actually drives, and engineers compare the two.
Removing the safety driver
The decisive step is taking the trained employee out from behind the wheel. Companies do it gradually: an employee moves to the passenger seat, then rides in the back, then the car drives empty, then it carries employees, then the public. Each stage generates the evidence regulators ask for. Waymo took about a decade from its first public-road tests to driverless rides for the public; a serious incident, as Cruise learned in 2023, can reverse the process overnight.
Frequently asked questions
How many miles has Waymo driven?
Waymo has reported more than a hundred million miles of fully driverless operation, and far more in simulation. The number grows by millions a month as its services expand, so any figure is out of date quickly; its own safety-data pages carry the current one.
What is a safety driver?
A trained employee behind the wheel of a test vehicle, hands near the wheel and foot near the brake, ready to take over. Most services begin with safety drivers and remove them only after testing shows the software handles the area alone.