Waymo LLC, Alphabet Inc.’s self-driving car division, has been testing its vehicles on public roads for a few years, and this week the company announced that its cars have already traveled more than 4 million miles.

Waymo began as an internal project at Google LLC in 2009, and in 2012 one of its vehicles became the first self-driving car to receive a legal license plate, allowing it to share the road with human drivers. Since then, Waymo’s public tests have grown to include 23 cities across the U.S., which include highly populated areas such as the San Francisco Bay Area and Phoenix.

The company plans to add other cities in the future, and it announced in October that it will begin testing its cars in wintry road conditions in Michigan.

In addition to public tests, Waymo has also continued experimenting with its cars on private roads, where it attempts to recreate real world scenarios that are either too rare or too dangerous to test in public. Waymo said its private tests have included 20,000 scenarios, from aggressive drivers “barreling out of driveways” to people jumping out of bags or lying down on moving skateboards in the road.

Waymo also tests its cars in simulations, where its vehicles have already driven more than 2.5 billion virtual miles, a distance roughly equivalent to driving to Mars and back more than eight times.

Although 4 million public miles is an impressive accomplishment for Waymo, the more telling figure is how fast that number is increasing. It took Waymo about 18 months for its cars to travel their first million miles on public roads, but they passed their last million milestone in just six months.

This shows not only how big Waymo’s fleet has grown in the last few years, but also how fast self-driving car technology is advancing, as the vehicles become safer and capable of driving longer distances while sharing the road with human drivers. Many of these advancements come from Alphabet’s own work with machine learning and artificial intelligence.

“With this accelerated learning cycle, we’ve been able to teach our vehicles the advanced driving skills necessary for full autonomy,” the Waymo team said in a blog post. “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.”

The ability for self-driving cars across an entire city, not just along one fixed route, will give more people quicker access to the benefits, the team added. “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.”

Earlier this month, Google announced that it would be launching a ride-hailing pilot program in Arizona, allowing members of the public to order a ride in a Waymo vehicle with no driver. Google said a Waymo employee will ride in the back seat of each car at the start of the program, but eventually the cars will be completely unmanned.

Photos: Waymo