Small, wheeled drones are delivering packages in some cities—but, like driverless cars, they need a helping hand a lot of the time.

The robots: Starship Technologies has six-wheeled robots that deliver food in places like London and Silicon Valley. Using nine cameras, four radars, eight ultrasonic sensors, and a bit of AI, they navigate their way along sidewalks like mini autonomous cars.

Their limits: At the AI Congress in London today, Starship’s Kristjan Korjus noted that the firm’s robots cost $5,500, and the aim is for them to cost $2,250. That pricing, he says, means “every sensor is quite crappy.”

Why that matters: There’s a limit to what cheap computers and sensors can do, even if the robots travel at just four miles per hour: currently, they can cross a road independently just 50 percent of the time. Korjus says Starship employs 100 operators to remotely control the robots during tricky situations, and that the firm won’t have fully self-driving bots any time soon.