Isaac Arnsdorf:

Here’s how they’d use more energy than conventional cars: more trips. In most households, each adult commutes, runs errands and shuttles the kids separately, according to the U.S. National Household Travel Survey. A self-driving car would make more trips to finish the same tasks, the University of Michigan researchers said. It might drop off one parent at work, return home to pick up the other, and then take the kids to school, return home, then start the return cycle.
 What isn’t known yet is how many people who don’t currently drive, like kids and users of public transportation, will start sharing a self-driving car. Those new trips — and all the return trips in between — could mean more total driving.