After quietly testing its fleet of self-driving minivans in Phoenix, Ariz., Waymo, Alphabet's self-driving car unit, is expanding its testing to hundreds of "early riders" in the area.

The details: Local residents can apply to be part of Waymo's trial program, which will give them access to the company's fleet of cars anytime they need a ride via a mobile app, ideally thinking of the cars as they would a primary and or secondary car. They'll be able to take rides within an area twice the size of San Francisco. Waymo also plans to add an additional 500 self-driving Chrysler Pacifica minivans to the 100 cars already in the fleet. A safety driver will be behind the wheel at all times, though the cars will drive themselves.

Waymo is also testing its cars in Mountain View, Calif., Austin, and Kirkland, Wash.