Passenger-mile vs vehicle-mile is a useful metric (you can add driver-mile if you want, or cost-mile) but you can probably do something similar with some measur

Passenger-mile vs vehicle-mile is a useful metric (you can add driver-mile if you want, or cost-mile) but you can probably do something similar with some measurement of storage space, too. A full minivan ranks surprisingly high on all of them, which is one of the reasons it's disappointing that carpooling and such aren't discussed as much anymore.