Self-driving car startup Zoox won permission Friday to start offering rides to passengers in California.

The Public Utilities Commission granted Zoox the first permit to ferry riders in autonomous vehicles under a pilot program. But Zoox can't charge for the service and a backup driver must be in the car.

Under the program, Zoox will submit information to the commission on the number of passenger miles traveled in the test cars and any incidents that occur, along with pollution and safety data.

There was no immediate word on when Silicon Valley-based Zoox would start its service, which would likely be confined to the San Francisco Bay Area. Messages to Bert Kaufman, the company's head of corporate and regulatory affairs, were not immediately returned.

It's a milestone in the race to make autonomous vehicles safe and comfortable enough to be trusted by lawmakers and the public at large. Besides the enormous technical challenges, companies must fight the backlash that can follow test car crashes, such as one last year in Tempe, Arizona, where an Uber autonomous vehicle struck and killed a pedestrian.

Zoox last week released its first safety report, which calls safety foundational to the firm, which is working on both hardware and software for self-driving vehicles.

More than 60 companies have California permits to test autonomous vehicles. But so far they can only carry employees and researchers.

In October, Google's self-driving car spinoff, Waymo, became the first firm to receive permission to test autonomous vehicles in California without a backup driver. Waymo, which has been working on self-driving vehicles for a decade, also has requested the Public Utilities Commission's permission to start a passenger service.

Earlier this month, Waymo began offering a small paid ride-hailing service in Arizona, where regulations are looser. However, those cars have backup drivers.

The service, dubbed Waymo One, is confined to a roughly 100-square-mile area in and around Phoenix.

General Motors also is gearing up to begin offering a ride-hailing service through its Cruise subsidiary. Cruise plans to start its ride-hailing service at some point next year in at least one U.S. city. Another self-driving car company, Drive.ai, has been giving short-distance rides to all comers within Frisco, Texas, and Arlington, Texas, since the summer.