No matter who you ask, the near-future of delivery seems to involve fleets of robots shuffling packages from stores, down sidewalks, and onto doorsteps. Robots will lug grocery bags from market to kitchen; they’ll begin to replace humans delivering take-out and dropping off parcels. And soon, your Amazon Prime packages may show up courtesy of Scout, Amazon's new six-wheeled autonomous delivery robot built to withstand the sidewalk.

Amazon announced on Wednesday that it will begin field testing Scout in Snohomish County, Washington, with Prime customers who request same-day, one-day, or two-day delivery. For now, Amazon says, it will limit its testing to daylight hours Monday through Friday, when sidewalk traffic is lowest. Scout will also have a human chaperone to make sure the bot can “safely and efficiently navigate around pets, pedestrians, and anything else in their path.”

For years, Amazon has experimented with ways to make its delivery cheaper, faster, and more efficient. Many of those efforts have focused on the “last mile” from a transportation hub to your home, when parking a car and dispatching a human takes precious time and energy. The company’s Delivery Service Partners incentivizes people to create their own logistics businesses that will work with Amazon. Amazon Prime Air, a drone-delivery initiative, has reportedly been in the works since 2013. An autonomous sidewalk rover would give the company a new method to shepherd packages for cheap.

But Amazon is very late to this ground-based delivery robot game, which is crowded with many players that already have years of real-world experience. Starship, for instance, launched in 2014 and has banked 150,000 miles of driving as of this month. Marble, a San Francisco startup, was delivering falafel for Yelp Eat24 when we profiled them in the spring of 2017. And Kiwi robots have been busy delivering food around the UC Berkeley campus—and, occasionally, catching on fire.

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Catching on fire is just one of many concerns for these robots. Think of them like self-driving cars, only they have way more nonsense to worry about. Roboticists consider roads to be relatively “structured” environments—they’ve usually got stoplights and neat lanes and signage. But sidewalks are pure chaos. A delivery robot has to dodge humans streaming out of storefronts and obstacles like buskers. (Depending on the maker, the robot might use sensors like machine vision or lidar.) On top of that, they have to then briefly navigate roads when they cross streets. It’s an incredible challenge in these early days of advanced robots that have begun escaping factories and labs to roam the real world.

Pull it off, though, and the rewards could be big. Delivering packages is still a very inefficient process, especially the last-mile segment: put package on truck, drive truck to neighborhood, make poor delivery man schlep each package one by one. The promise of delivery robots is to automate that last bit, which will theoretically save companies like Amazon money, on top of keeping emissions from idling trucks out of the air.

Still, though, this is such a new space that the economics of it haven’t been fully worked out. “It's a market that has yet to be validated,” says Brian Gerkey, CEO of Open Robotics. “You've seen a lot of startups working on this, but I think there's always been this question of whether it's going to make economic sense. It's tough to beat the capabilities of a person who goes around doing that last-mile delivery.”

A robot never tires, but humans are still far more capable at virtually everything. A delivery robot can’t open gates without hands, and it can’t climb steps to get right to your door. And if the robot requires the customer to enter a PIN to get the package out, how can the robot leave the package if you’re not home? “Also, to be able to react to unexpected situations,” Gerkey says. “People are just going to be fantastic at that.”

With Amazon Scout, you can see above that it moves at about a walking pace. “That's smart of them, because it doesn't go any faster than a human walking. That may make it a little more accepted,” says Bob Doyle, vice president of the Robotic Industries Association. “It's not doing wheelies or anything.”