The voluntary recall comes as scrutiny by federal regulators and local school district officials increases.

The inclusion of the Waymo look-a-likes appears to be part of a larger storyline that will encourage players to "stop the development of a mass surveillance network."

The agency already opened an investigation in October over Waymo's performance around school buses. Waymo said it issued a software update to its fleet to improve performance.

Baltimore, Philadelphia, Pittsburgh, and St. Louis join a long list of cities where the company is gearing up to offer robotaxi rides.

The service comes with a few caveats, including the addition of a human safety operator behind the wheel and a limited operating area. The companies said fully driverless operations, without a safety operator, will begin in the future and the service area will expand.

The Chinese autonomous vehicle company Pony.ai has global ambitions and plans to build a 3,000-strong robotaxi fleet to meet them.

The new cities will see Waymo tackle harsh winter weather and narrow streets.

The permit is the final regulatory step to launch a robotaxi service in Arizona, a state that has become a hub of autonomous vehicle technology testing and development.

Waymo said it will do the same in Dallas, Houston, San Antonio, and Orlando in the coming weeks -- the latest steps in the company's continued expansion across the U.S.