Google’s driverless car division Waymo has reached the milestone of simulating a daily total of 20 million miles of autonomous driving on virtual roads as it tries out new scenarios for its software.

SiliconAngle.com reports that the testing replicates congested roads, intersections and other challenging driving environments with engineers able to tweak these simulations to make things even more difficult, speeding up traffic or placing extra cyclists between virtual vehicles.

“Not every mile is created equally,” the Waymo team says in a blog post. “A driver has to make fewer decisions while driving on a highway than the bustling streets of San Francisco. In simulation, we can pick the most interesting encounters from our over 20 million autonomous miles on public roads in 25 cities across the country to maximise the efficiency of our learning.”

The daily testing is around 100 years’ worth of driving on real roads.