The company, which started its self-driving car program in 2009, says in a monthly report on Monday that children helped it test the vehicle’s sensors and software. Google had children at the company’s Mountain View headquarters to stand around its parked cars to give the sensors and software extra practice.

We teach our cars to drive more cautiously around children. When our sensors detect children—costumed or not—in the vicinity, our software understands that they may behave differently. Children’s movements can be more unpredictable—suddenly darting across the road or running down a sidewalk—and they’re easily obscured behind parked cars.

Google introduced the prototype—a gumdrop-shaped vehicle it designed itself—in June 2015. The self-driving car doesn’t have pedals or a steering wheel, but only sensors and software. It hopes to commercialize its technology by 2020. The company’s tests still include Lexus RX450h SUVs equipped with autonomous software.

The October monthly report provides some insight into why Google pursued fully autonomous vehicles in the first place—as opposed to semi-autonomous ones that require some driver engagement. In 2012, once Google’s software had progressed enough, the company found volunteers (all employees) to drive its Lexus vehicles on the freeway portion of their commute.

After drivers had got on the freeway, they could turn on the self-driving feature. Google’s self-driving team told the volunteers to pay attention at all times because it was early-stage technology and they needed to be ready to take over at any moment.

That didn’t exactly happen though. Volunteers trusted the technology quickly and were captured via video engaging in all sorts of distracted driving behavior, including turning around and searching for an item in the backseat.

Google wrestled with the challenge of how to keep drivers engaged enough that they can take control of driving as needed. In the end, the company opted to bypass that problem and make fully autonomous vehicles instead, Google says in its report.

A few other stats from Google’s October self-driving car report:

• No accidents were reported in October.
• Google has 23 Lexus RX450h SUVs on public streets; 19 are in Mountain View, Calif., and four are in Austin, Texas.
• Google has 25 prototypes of its own self-driving car on public streets; 21 are in Mountain View and four are in Austin.
• Since 2009, Google’s drivers have traveled nearly 1.3 million miles in autonomous mode, which means the software is driving the vehicle and test drivers are not touching the controls.
• Since 2009, drivers have traveled just over 900,000 in manual mode.
• Google currently averages between 10,000 and 15,000 autonomous miles per week on public streets.