HARRISBURG, Pa. (WHTM) — Driverless trucks are coming. But where and when?

Start with where: They’ll be on interstate highways long before other roads, said Noël Perry, a transportation economist who speaks internationally — most recently this week at a conference in Indianapolis — but is based in Cornwall Township, Lebanon County.

The reason?

“There are no pedestrians, which is a big problem for automation,” Perry said. “It means that “Everybody is going in the same direction on one side of the road and the other. And there’s controlled access, so you don’t have to worry about somebody coming in and running a stop sign.”

As for when, Perry said the world won’t suddenly flip a switch and go from manual to automatic.

“You could have trucks going one after the other with a driver in front, controlling the rest,” he said. “So it’ll be a gradual thing, but we’ll start noticing it within five or six years.”

Full automation, Perry said, could happen closer to 2045 — and driverless cars will generally precede driverless trucks, he said.

Perry said actually, automation has been happening for a long time.

“Almost everything the driver did in 1920 is automated,” he said. You don’t have to crank the truck to get it started. You don’t have to get a map out and figure out where you are.”

The benefits are automated trucking, Perry said, are clear: It’ll be safer, he said (because robots don’t drive tired or under the influence of substances or distracted by cell phones) and far cheaper, because trucks won’t have to be idle and unproductive when drivers sleep and because companies won’t have to pay as many skilled drivers.

That, of course, is where the tradeoffs start. Trucking employees millions of people nationally, and the industry is particularly important in central Pennsylvania. Perry and other experts expect longhaul trucking jobs to be particularly impacted by automation.

“I would not bet on a 30-year career as a truck driver right now,” Perry said. “Now, you can make a lot of money in the short term, and they sure need people. But 20 years from now, those jobs will be gone.”

A spokesman for the Teamsters union, which represents many truck drivers, didn’t respond to messages seeking comment.

Shorter-haul trucking — driving loads from highway terminals, for example, into towns and cities and to front doors — could be more resilient. If automation is inevitable, what’s the best way to help workers?

“The important part is that we have a policy that ensures that local truck driving jobs are good, that workers aren’t misclassified as independent contractors and that they’re paid for all the time that they work,” said Steve Viscelli, an economic sociologist with the University of Pennsylvania.

Perry said American workers have absorbed similar disruptions before, like the automation of farming between the 1920s and 1940s; during that time, farming went from employing nearly half of Americans to single-digit percentages, he said, but overall employment grew.

Still, “there is a painful period of adjustment because they have to go find a new job and the economy has to create those jobs based on the new technology,” Perry said. In fact, he said, the time that took during the 1930s partly explains the Great Depression.

New supply-chain practices could make the Midstate even more important to shipping in the future than it is now, Perry said. That’s because wereas now, a tractor-trailer full of goods might go from Harrisburg to Boston and then be broken up there into smaller loads — so that only one driver is necessary for the long haul rather than several — several smaller automated trucks might leave directly from Harrisburg bound for different areas surrounding Boston.

“That would make the Harrisburg area much more important,” Perry said, “because we would be making small packages as well as big packages.”