Chris Urmson, a Carnegie Mellon University research scientist, joined Google in 2009 to help create the then-secret effort. He took over leadership of the team after Sebastian Thrun, the Stanford computer scientist and founder of Google X laboratory, left in 2013. Johnny Luu, a spokesman for Alphabet, the parent company of X, the company’s research division that oversees the car project, confirmed Mr. Urmson was planning to leave. “Seven years ago, the idea that a car could drive itself wasn’t much more than an idea. Chris has been a vital force for the project, helping the team move from a research phase to a point where this lifesaving technology will soon become a reality. He departs with our warmest wishes,” Mr. Luu wrote in an email message.