The most impressive thing about the Uber trip from the Midwest to Southern California wasn’t that the truck drove itself the 344 miles across Arizona. It was what happened when two men named Larry and Mark met at the western edge of the Copper State. Larry, the trained safety driver, had spent the autonomous voyage watching over his robot. Mark was freshly arrived from Los Angeles in a conventional truck.

Each unhitched their trailer and hooked up to the other’s. Mark drove his new pile of cargo to its final destination. Larry headed back east, flipping his semi into autonomous mode.

This recent meet and greet, which Uber described today, marks the ride-hailing giant’s latest step into the world of long-haul trucking: a step that depended not just on self-driving tech, but on the logistical firepower to make it work for real.

Uber has been talking about trucks since it acquired Anthony Levandowski’s self-driving big-rig startup, Otto, in 2016. (Between Levandowksi’s role at the center of Uber’s bruising legal fight with Waymo and a trademark lawsuit, Uber dropped the Otto name last year.) And it’s far from the only company trying to teach robots to drive semis: Volvo, Daimler, Waymo, Tesla, and several startups have declared similar intentions.

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That’s because trucking is, as they say in the Valley, ripe for disruption: Americans are shipping more and more stuff. There aren’t nearly enough drivers to move it all, and the shortage will only get worse over time as Amazon continues to disrupt (there's that word again) traditional retail stores. And while it’s hard to get vehicles to drive themselves in complex cities, the trucker’s domain—the freeway—is easy to master. Just stay in your lane and brake before you hit the guy in front of you.

The question is not, then, can the technology work, nor is it whether people will pay for it. It’s what happens when the truck leaves the highway and must negotiate intersections, avoid squashing pedestrians, and navigate through industrial yards. The consensus among the companies trying to take trucking robo is that all that stuff is simply too hard. So they have cooked up a few workarounds.

One startup, Starsky, wants to separate vehicle and human: People in call centers would remotely operate the trucks on surface streets. Uber is going with what we call the bar pilot model. A human in a regular truck picks up the shipment, takes it to a transfer hub next to the highway, and passes it over to the autonomous truck. The robot (with a licensed driver behind the wheel, for now) does the simple highway cruising. When the truck eventually exits, the handoff runs in reverse, and another human takes control for last few miles—like the bar pilots who lead ginormous container ships into and out of port. Someday, Uber’s transfer hubs could be dedicated facilities with their own on- and off-ramps. For now, its trucking team is using weigh stations off Interstate 40 in Topock and Sanders, Arizona, at either end of the state.

Uber's self-driving trucks use hardware and software developed by the company's R&D team in Pittsburgh, Pennsylvania. Uber

That’s the essence of it, but Uber imagines a much more intricate operation. This is where the meeting of Larry and Mark comes in: Each had a load of cargo to haul on each leg of their trip, so neither wasted time or money moving an empty box.

The point of Uber’s robo-trucking scheme isn’t just to safely get piles of sneakers and diapers and dog food from warehouse to freeway to warehouse. It’s to do it with logistical efficiency. When human drivers bring a shipment to the robotruck, Uber wants them to have a newly arrived shipment to bring back. The goal is to keep every vehicle, no matter who or what is working the gas and brakes, to be moving cargo—and making money—at all times.