MOUNTAIN VIEW – Google’s autonomous vehicle program has taken some big hits in recent days. Two of the project’s top engineers have reportedly left to work on a new start-up, but the biggest setback appears to come with the departure of Chris Urmson, the venture’s technology chief.

A pioneer in autonomous vehicle research at Carnegie Mellon University, Urmson joined Google in 2009 and was heading the program until John Krafcik, the former Hyundai Motor America CEO, took the helm a year ago.

“I’ve decided the time is right to step down and find my next adventure,” Urmson said in a blog post. “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.”

Along with Urmson, a number of other autonomous vehicle team members have left Google in recent weeks. The New York Times reports that includes two key engineers, Jiajun Zhu and Dave Ferguson, both leaving to work on a new start-up project.

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