Self-driving cars are still very much a technology of the future, however their presence in the here and now is growing. Alphabet’s Waymo is among the pioneers pushing self-driving vehicles out into real-world testing, and it has just revealed that its cars have passed the milestone of driving more than 4 million miles on public roads. The speed at which Waymo is accumulating its driving experience is accelerating nicely: the last million miles took six months, whereas the first million required a year and a half.

Waymo’s autonomous systems learn from a combination of experience on public roads and testing on the company’s private track, the latter of which is inspired by scenarios encountered out in an uncontrolled environment. At this point, more than 20,000 unique scenarios have been modelled at Waymo’s private test facility, which includes such rare and unlikely events as “people jumping out of canvas bags or skateboarders lying on their boards.”

For comparison, Uber’s self-driving efforts only just reached a million miles driven in September, putting the ride-hailing app company significantly behind in the experience stakes.

It's been 1 year since we launched self-driving Ubers in Steel City—here's to 1 million miles and 30,000 trips. pic.twitter.com/ayJhTAFUxx — Uber (@Uber) September 14, 2017

Earlier this month, Waymo began sending out self-driving vehicles without a safety driver on board, and the company anticipates being able to give people rides in those entirely driverless cars in the near future. Geographically, it’s all still focused on the western United States, however Waymo’s ambitions — bolstered by millions of real-world miles and billions of simulated miles of driving — are understandably far grander than its present scale.