PHOENIX — Wednesday marks the official debut of Waymo One, a new taxi service launched by Alphabet Inc., the parent company of Google. A group of 400-plus Phoenix-area early adopters will have 24-hour access to an unknown number of Waymo’s 600-unit fleet of Chrysler Pacifica Hybrids through a proprietary smartphone app.

Much like ride-hailing leaders Uber and Lyft, Waymo’s app allows users to store their credit card information, check availability and price, and contact the service. Unlike its competitors, Waymo’s vehicles navigate in “fully” autonomous mode — for the moment, every unit will be accompanied by a human driver tasked with intervening in emergencies.

Waymo units have been tested on public roads throughout the U.S., tallying more than 10 million miles, many of them in the Arizona cities of Chandler, Gilbert, Mesa, and Tempe.

In March, a Tempe woman was struck and killed by an Uber-operated SUV while walking her bicycle across the street at night. The Uber unit failed to stop and the vehicle’s “safety driver” failed to react in time. Despite that incident and others, Arizona remains a friendly regulatory environment for autonomous vehicle testers. No federal guidance on the technology appears to be immediately forthcoming.

“Over time, we hope to make Waymo One available to even more members of the public as we add vehicles and drive in more places,” wrote CEO John Krafcik in a blog entry Wednesday. “Self-driving technology is new to many, so we’re proceeding carefully with the comfort and convenience of our riders in mind.”