Elon Musk has dropped his latest vision on us — a future in which we avoid "soul-destroying" traffic by vanishing underground in our electric, self-driving cars, and tool through our cities within subterranean tunnels, only to pop out at our destination on the surface. Musk says such tunnels will be necessary, namely because we are about to see a massive increase in the number of cars on the road.

Not so fast: A professor at Carnegie Mellon University is pouring cold water on the idea, at least for now. Costa Samaras, informed by his work on the mile-long extension of New York's No. 7 subway line in the mid-2000s, suggests that technologists already have enough on their hands optimizing fully autonomous cars; it's getting way ahead of themselves to contemplate yet another ultra-complex concept. He told me:

Maybe let's figure out shared, fully-driverless, electric mobility first?

Here's a video that Musk played while speaking on stage with TED's Chris Anderson on April 28.

Musk's calculus: Most people, he said, think that once all or most of us are zipping around in robot cars, everyone will be able to drive faster with much less road congestion. But that is almost certainly wrong, he said. With shared driving, "the affordability of going by car will be better than that of a bus. It will cost less than a bus ticket. So the amount of driving that occur will be much greater with shared autonomy, and traffic actually will get far worse."

This is where I asked Samaras' opinion. He noted that Musk's video shows not only tunnels, but giant caverns that would require expensive excavation and ventilation and automated elevator shafts, all of which would need to be reliable. "By my count, there are at least twenty plain old passenger elevators out of service in the New York City subway system today," he said. We are talking big money, and lots of time. That one-mile extension of the No. 7 line on which Samaras worked? It cost $2.5 billion and took a decade.