What’s the difference between delivering passengers and delivering parcels?

Boston residents learned this week that a “pop up mass transit” system called Bridj was experimenting with delivering parcels as well as passengers, and could soon use neighborhood lockboxes and mobile robots to carry each package to its final destination.

“We’re going to start to introduce autonomous vehicles and autonomous delivery devices over the coming months, using Boston as a laboratory,” the firm’s 26-year-old cofounder and chief executive, Matthew George, told the Boston Globe.

Commuters in Boston, Washington, D.C., and Kansas City can already use a smartphone app to summon one of Bridj’s 14-person mini-buses, which use routing software to calculate malleable, on-the-fly bus routes. Like a shared ride on the taxi-alternative service Uber Technologies Inc., the two-year-old business is touted as an alternative to typical mass transit options like fixed-route commuter buses or subway networks.

But just as Uber has extended its ride-hailing system to last-mile parcel delivery, Bridj is also exploring ways to put boxes as well as butts in its vehicles’ seats. In a twist on other last-mile parcel delivery services, Bridj plans to use its passenger vans to drop items at local lock-boxes, then deploy either human couriers or small robots to carry each box to its final street address.

The idea faces several hurdles before it takes off in practice, since Bridj hasn’t yet chosen a delivery robot from its experiments with manufacturers, or obtained permission from the city of Boston to drive the autonomous critters down urban sidewalks. Another specter is potential vandalism or cargo theft committed on the slow-moving bots.

Despite these challenges, Bridj’s ideas on ways to improve last-mile delivery are hardly unique. In recent months, inventors in both Israel and in England have unveiled similar plans to dispatch packages to urban locations in wheeled autonomous robots.

The concept of using centralized lockers as distribution hubs for urban neighborhoods is even more popular, with pilot projects underway by some of the industry’s biggest names, including Atlanta-based supply chain giant UPS Inc., Durham, N.C.-based Bell and Howell LLC, and German transport and logistics firm Deutsche Post DHL Group.

Bridj hopes to emerge from this pack of parcel-toting hopefuls by building its service from a mixture of all three popular ideas—ride hailing, central lockboxes, and robot delivery. Only time will tell if the creative recipe works.