When it comes to developing self-driving cars, experience is key to making the technology safe and ready for the road. That’s why we’ve been working to make Waymo the world’s most experienced driver. Now, Waymo’s fleet of vehicles has officially reached more than 4 million self-driving miles on public roads (for comparison, that would take the average American driver nearly 300 years to complete). These millions of miles of experience have been crucial to reaching our latest milestone: putting the world’s first fully self-driving cars on public roads without anyone in the driver’s seat.

With these 4 million miles of experience, we’ve been able to focus on varied and complex driving scenarios. For every situation we encounter on the road, we’re able to amplify and multiply the experience in simulation and on our private test track.

Here’s a look at a few more numbers that has made Waymo the most experienced driver on the road:

6: Months it took to accumulate our last million miles, compared to about six years for our first million. We continue to accelerate our real world driving and our pace of learning.

23: The number of U.S. cities where we’ve tested our self-driving vehicles. We’ve driven in San Francisco and every bridge in the Bay Area, the hills of the Santa Cruz mountains, and the dust storms of Arizona.

20,000: Unique scenarios conducted on our private test track, inspired in part by what we see on public roads. At our 91-acre private testing facility, we practice rare and unusual situations, such as people jumping out of canvas bags or skateboarders lying on their boards. We can also prepare our cars to handle some of the most common scenarios that lead to collisions, such as aggressive drivers barreling out of driveways.

2.5 billion: Miles driven in simulation in the last year alone. Every day in simulation, more than 25,000 virtual self-driving cars practice hard cases we’ve already encountered on the road, or re-drive all the miles we’ve ever driven so we can dramatically accelerate our pace of learning.

With this accelerated learning cycle, we’ve been able to teach our vehicles the advanced driving skills necessary for full autonomy. We’ve been able to unlock an entire geographic area for our fully self-driving cars, and soon members of the public will get to use Waymo’s driverless service to go to work, to school, to the grocery store and more. Unlocking self-driving cars across an entire city, rather than simply traveling along a single fixed route, will let more people access and benefit from this technology, sooner. When fully self-driving vehicles become a part of people’s daily lives, we can move closer to our goal of making transportation safe and easy for all.

For more on our testing program and the steps we’ve taken to prepare our cars for full autonomy, see Waymo’s Safety Report.