Stewart may be a particularly good fit. As our TechCrunch colleagues observe, his expertise revolves around short-term travel rentals. Before Airbnb, he was the CEO of the luxury vacation deal site Jetsetter -- he knows a thing or two about getting these services off the ground. And that's important when the concept of a driverless taxi is still brand new, with plenty of nuts-and-bolts details that need to be pinned down in the years ahead.