Waymo—Alphabet's self-driving car division—is launching a "trial" of a self-driving taxi service in the Phoenix, Arizona, metropolitan area. The Google spinoff's fleet of self-driving cars is descending on Phoenix and offering free rides to anyone in its "early rider program," which is currently accepting new members.

The taxi service is not totally "self-driving." Waymo notes that "as part of this early trial, there will be a test driver in each vehicle monitoring the rides at all times." While the car will handle most of the driving duties, a driver will ensure nothing goes wrong if the car runs into a situation it can't handle. While the trial will offer free rides to Phoenix residents, it will also serve as a research program for Waymo. The company's blog post say it wants to "learn things like where people want to go in a self-driving car, how they communicate with our vehicles, and what information and controls they want to see inside."

To handle the load of a city-wide taxi service, Waymo is building 500 more of its self-driving Chrysler Pacifica Hybrid minivans , bringing the total minivan fleet to 600. The minivans represent the latest in Waymo's technology. In a recent talk at the North American International Auto Show, Waymo CEO John Krafcik said the vehicles would be the launch platform for Waymo's "full-stack approach," which combines Waymo's software with a " fully integrated hardware suite " that is "all designed and built, from the ground up, by Waymo." Most self-driving car programs stick to developing software using Velodyne's LiDAR hardware

Arizona is an ideal place to launch the self-driving taxi program, since it doesn't have much in the way of specific self-driving legislation. The state requires only that a driver be present in the car and take responsibility for adhering to the existing rules of the road. Other states, like California, require special licenses, special insurance, and public reports of every accident a car is involved in.

The move is the latest in a battle between Waymo and Uber as both race to develop a self-driving taxi service. Uber—which currently has 43 self-driving cars—hopes to transition to a self-driving fleet, which would allow it to cut out Uber drivers, slash costs for customers, and maybe even earn a profit. After Uber hired a former Waymo employee, Anthony Levandowski, Waymo sued Uber, accusing Levandowski of stealing 14,000 confidential documents while he worked at Waymo.

In addition to fighting Waymo, Uber's rival Lyft has partnered with GM on the development of self-driving cars. No company quite compares to the scale of Waymo, though. The company's 600 minivans would bring its total to 680 self-driving vehicles, which would make it the largest self-driving fleet in the world.