Why should there be any reduction? In the past, technological advancement has always led to higher employment, higher wages, and higher standard of living. Why are we so cynical that we expect different this time around? Note that this is isn't directly specifically at you, but at the general notion that automation can only displace workers, which seems to be pervasive.
You raise a valid point. The Industrial Revolution certainly didn't kill any large-scall economies, after all. I think there absolutely WILL be a significant reduction of jobs, even for automated vehicles alone. And yeah, some displaced workers will go into technician roles to support our new robot overlords, but I'm having trouble imagining new job creation on the scale of several million, in the U.S. alone. I wouldn't say automation can only displace workers, but I anticipate a net reduction of jobs, especially in the minimum wage and unskilled labor market. Do you not believe there will be a net reduction? And if you see millions of new jobs on the horizon, what are they?
I believe that many current industries will be obsolete. We need only to look to the fate of agriculture and rail roads to know that no industry, no matter how blue chip, is invincible to the advance of better technologies. There will no doubt be a giant reduction in the number of people who work as mailmen and dry cleaners. Hopefully even lawyers will be reduced. That said, if I knew where all of these jobs would be made up, then I'd be a fortune teller, or at the very least a very wealthy investor. I haven't a clue, but experience tells us that given the space to innovate, create, and improve, we will do so. It's less optimistic naïveté, and more inductive reasoning, but I'll not venture a guess about mechanism.