Waymo Tuesday added Washington to its pioneering robotaxi service's steadily expanding list of U.S. markets, although passengers will have to wait until next year until they can take a driverless ride around the nation's capital city.

For now, Waymo's robotaxis will continue to map Washington's streets and corridors with a safety driver sitting behind the wheel to take control of the vehicle if something goes wrong — a precaution required under the regulations currently in force in the District of Columbia.

That's something Waymo already has been doing since it began sending out its robotaxis in Washington in late January after a brief trial run in the capital last year.

While the robotaxis continue to learn their way around the city, Waymo executives expressed confidence they will be able to work with regulators to clear the way for completely driverless rides at some point next year through its Waymo One app.

“We’re excited to bring the comfort, consistency, and safety of Waymo One to Washingtonians, those who work and play in the city every day, and the millions of people from around the world who travel to the District every year," Waymo co-CEO Tekedra Mawakana.

If Waymo's ambitions pan out, Washington and Miami next year will be added to four other U.S. markets where its robotaxis are transporting passengers — Phoenix, Los Angeles, the San Francisco Bay Area and Austin, Texas as part of a partnership with ride-hailing leader Uber. Waymo and Uber also are teaming up to begin dispatching its robotaxis in Atlanta later this year.

The growth has helped turn what began as a head-turning novelty in Phoenix and then in San Francisco into an increasingly common sight in the cities where Waymo operates. The company says it had provided more than 4 million driverless rides to paying customers through the end of this year, and is now providing them at a pace of 200,000 paid trips per week.

That has established Waymo as the early frontrunner in driverless technology while others are racing to catch up. Both Amazon and Tesla are gearing up to launch their own services in different U.S. cities while another ride-hailing service, Lyft, has announced plans to add robotaxis as an option in Atlanta and Dallas.

Waymo's early lead in the still-nascent robotaxi market is a vindication of a technology that began as a secret project within Google in 2009 before it was spun off into a separate company owned by Alphabet Inc. in 2016.