Advanced Technology Group leader Eric Meyhofer wouldn't give a specific start date but he said Uber won't deploy the driverless cars without human backups unless they are proved safe.

"Once we can check that box, which we call passing the robot driver's license test, that's when we can remove the vehicle operator," Meyhofer said in an interview at an auto industry investors conference Detroit. "We're going aggressively too."

Waymo, the name of the autonomous car unit of Alphabet Inc.'s Google, currently is testing on public roads in the Phoenix area without human backups and plans to carry passengers soon. General Motors Co.'s Cruise Automation has promised to start sometime next year in an unspecified location.

Meyhofer said Volvo XC-90 SUVs are being prepared for the work. Uber Technologies Inc. has 215 test vehicles carrying passengers with human backups in Phoenix, San Francisco, Pittsburgh and Toronto. They travel 80,000 miles per week gathering data and have given 50,000 paid rides, he said.

San Francisco-based Uber started carrying passengers with autonomous vehicles in 2016, first with two Uber employees on board, Meyhofer said. The person in the passenger seat documented the vehicle's behavior while the one on the driver's side would intervene if the car needed help. The vehicles have improved to the point where Uber has removed one of the humans and now the backup driver enters data on a screen in rare cases, Meyhofer said.

The ride service now has 1,600 people working on autonomous vehicles in the four test locations.

Waymo said in November that it plans to carry passengers without human backups in a few months but gave no definite date. The testing would be done in a relatively small area that is covered by detailed three-dimensional maps.

Meyhofer said Uber would start in the same way, gradually expanding the size of the area as mapping is done and vehicles become more capable. Currently the cars are limited to 40 miles per hour, Meyhofer said.

Such testing likely will be done in warm-weather areas that don't get snow. Meyhofer said Uber is still working on use of the cars when snow covers the lane lines and can't be seen by cameras. "That's all well within our scope. It's just not in our scope today," he said.

The developments on the road come as self-driving car technologies are being battled over in court. Waymo has alleged that one of its top self-driving car engineers, Anthony Levandowski, stole its trade secrets before founding a startup that he sold to Uber for $680 million. Uber has denied the allegations