Waymo also recently launched a commercial robotaxi service in Austin, Texas, just in time for the city's annual South by Southwest festival.
While would-be competitors including Elon Musk's automaker Tesla, and Amazon-owned Zoox, are continuing their own robotaxi testing and development, Waymo has pulled far ahead of self-driving companies in the U.S.
Before Tuesday's expansion, Waymo said it was serving more than 200,000 paid trips per week across San Francisco, Los Angeles and Phoenix.