Commercial operations have begun in the metro Phoenix area

The Waymo One self-driving taxi service has been given a soft launch in and around Phoenix, Arizona.

The service is currently limited to people on the company's Early Rider program. Waymo says it will "gradually roll out [the service] so even more people can ride with us", although it hasn't provided a timeline.

Booking a ride works almost exactly as it does on Uber and other ride-sharing services. Simply fire up the Waymo One app, confirm the pickup location, set a destination address, and tap Request Ride.

Using the company's fleet of Chrysler Pacifica plug-in hybrid people movers, each self-driving taxi can accommodate up to three adult passengers and one child.

For now, a trained Waymo employee will sit in the driver's seat to provide backup and assistance. Riders can also have their questions answered via the app or one of the in-car touchscreen units.

Cars are available 24/7 within the service area, which currently covers Phoenix and nearby cities, including Chandler, Gilbert, Mesa and Tempe.

Waymo started life as the Google self-driving car project, and was spun out into its own business unit in December 2016.

According to the company, its self-driving system has accumulated around 16 million kilometres of experience on public roads.

During the Early Rider trial program, which was launched in April 2017, riders have used Waymo's service to travel to school, college and work, as well as accompany vision-impaired relatives on shopping trips, go to weekly book club meetings, connect with public transport, and visit new restaurants.