Waymo’s collective odometer has reached four million self-driven miles using its fleet of self-driving cars on public roads.

According to the company’s latest blog post, published on Medium, its self-driven mileage pace has grown exponentially, with less time being taken in between each million-mile milestone.

Using a colorful line graph, Waymo reveals it needed 18 months to achieve the first million-mile milestone, with 2 million miles hit 16 months later, 3 million miles seven months after that, and 4 million miles clocked this month. The 4-million mile mark arrives after Waymo has completed testing in 23 U.S. cities of varying topographies to better capture data.

Other achievements cited include 20,000 unique, tested scenarios using its private test tracks and 2.5 billion simulation miles, which was achieved by 25,000 “virtual” self-driving cars conducting test rounds using actual real-world environmental factors.

The purpose of Waymo’s self-driving trials is to increase its autonomous system’s learning capabilities and refine algorithms, adopting its technology to respond appropriately to many driving scenarios in an open environment. Using Waymo’s private test facility, the brand has managed to replicate thousands of scenarios from the road, as well as odd instances, such as people jumping out of canvas bags.

As for next steps, Waymo is planning on inviting the public to test its new driverless service in the Arizona region. Phoenix and Chandler will serve as its initial service areas for a ride-hailing service, complete with a mobile app. These experimental vehicles will have a Waymo employee in the back seat to override settings in the event of an emergency.