Michael Sivak and Brandon Schoettle at the University of Michigan Transportation Research Institute (UMTRI) looked at patterns of car usage in American households and concluded that self-driving automobiles with a “return-to-home” mode “could reduce the number of vehicles needed within a single household by allowing sharing of vehicles in situations where it is not currently possible.” The idea is that the drivers in many households have little “trip overlap,” the term used to describe times during the day when more than one driver needs a vehicle. In households where there isn’t much overlap, a self-driving vehicle could, for example, bring one spouse to work early in the morning, then return home on its own to pick up the other in order to make a separate run to a different workplace, drop off kids at school, help run errands, etc., before retrieving that original dropoff at the end of the work day.