SAN FRANCISCO – Google has lost the man sitting in the technological driver’s seat of its pioneering self-driving car project.

In a blog post on Medium Friday, chief technology officer Chris Urmson announced that effective immediately he was leaving the team after shepherding Google’s pioneering effort through seven years and 1.8 million miles of road testing.

“I’ve decided the time is right to step down and find my next adventure,” Urmson wrote. “After leading our cars through the human equivalent of 150 years of driving and helping our project make the leap from pure research to developing a product that we hope someday anyone will be able to use, I am ready for a fresh challenge.”

Urmson did not indicate what he would be doing next. The news comes as The New York Times reports citing unnamed sources that two Google car engineers, Jiajun Zhu and Dave Ferguson, have left to work on a start-up.

Johnny Luu, a spokesman for Alphabet, the parent company of the “moonshot” X division of Google, would not comment on the departure of the two engineers.

But Luu added that “Chris has been a vital force for the project, helping the team move from a research phase to a point where this life-saving technology will soon become a reality. He departs with our warmest wishes.”

Google car CEO John Krafcik tweeted that “Chris is an incredible colleague & leader. Thank you for your passion & humility. Good luck on your new adventures!”

The roboticist’s departure is significant. Urmson is a leading light in the once niche and now burgeoning field of autonomous driving, having honed his craft at Carnegie Mellon University and through various Department of Defense self-driving car contests.

Experts such as Urmson ncreasingly are the life blood of an array of private efforts to build semi- and eventually fully self-driving vehicles. Companies pursuing this mission include tech outfits such as Uber and reportedly Apple, as well as a growing number of auto manufacturers including Audi, BMW and Ford.

One of the most advanced if controversial efforts is being pushed by electric automaker Tesla, whose semi-autonomous Autopilot feature has logged some 100 million miles but has also resulted in a death of a Florida Tesla owner, which regulators are investigating.

Google’s effort in the field stands out because the company remains determined to produce a self-driving car that seats two people and does not feature a steering wheel or pedals. Most other automakers are choosing to retain those critical components of the traditional automobile.

Most experts believe that while the technology to create an autonomous vehicle is developing quickly, what will lag behind is both government regulation and public acceptance of the transition from human-piloted machines.