Robot cars are now officially a real business. Waymo on Wednesday launched a commercial robot ride-hailing service in Arizona called Waymo One.

Like Uber or Lyft, customers will summon a ride with a smartphone app. But in this case, the car will be driving itself.

"This is a game changer. It's historical in nature," said Grayson Brulte, who heads driverless car consulting firm Brulte & Co.

Only "a few hundred customers" will have access to the app and participate in the early stages, according to Waymo, which is an arm of Google parent Alphabet Inc. Although the cars will drive themselves, a Waymo engineer will sit behind the wheel in case anything goes wrong. Waymo did not say when the cars will start arriving without a human minder or when the program will be expanded.

Waymo's cars, Chrysler Pacifica minivans bristling with autonomous driving technology, are available in several eastern and southeastern Phoenix suburbs, including Chandler, Tempe, Mesa and Gilbert. The fares are similar to those charged by Uber and Lyft.