SAN FRANCISCO, July 20 (Xinhua) -- Google's sister company of autonomous car business Waymo announced Friday that its self-driving cars have hit a new milestone with a record 8 million miles (about 12.8 million kilometers) on public roads.

"Our cars have been on the move! Waymo has now self-driven over 8 million miles on public roads across 25 U.S. cities, and over 5 billion miles (about 8 billion kilometers) in simulation," Waymo tweeted Friday.

The self-driving unit of Google's parent company Alphabet said the new figure doubles the total number of miles that it has traveled since 2017.

Waymo's self-driving cars are now traveling 25,000 miles (about 40,000 kilometers) every day, which is twice what the average American drives in a whole year, the firm said in a Facebook post.

"We're building the world's most experienced driver and preparing to launch our fully self-driving service," Waymo said on Twitter.

Waymo's new record reflects that its early investment in self-driving technology has paid off and dwarfed its rivals by a huge margin.

Last year, Waymo said it had driven 4 million miles (about 6.4 million kilometers) on public roads, two times more than the number in eight months.

Waymo began as a Google self-driving car project in 2009 and has now grown into an independent autonomous car technology firm under Alphabet. The company currently operates more than 600 self-driving minivans running on public roads.