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Google is letting people use its driverless car service for any ride at any time.

The search firm's sister company Waymo has created a free "early rider" programme in Phoenix, Arizona.

Hundreds of families are expected to take part. Waymo has equipped a fleet of 500 minivans with its self-driving technology to handle ride requests.

The company said testers could ride any time across a test zone in Phoenix twice the size of San Francisco.

The test is the first, large-scale public trial of a driverless car system.

Waymo's chief executive, said it had been doing small-scale tests of its riding service with a few Phoenix families for the past month.

Now, he said, it wanted more testers, with "diverse backgrounds and transportation needs".

"We'll 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," wrote Mr Krafcik.

Those applying to take part must be over 18 and live inside the large test region, which forms part of the greater Phoenix metropolitan area.

The cars will not be entirely autonomous, as Arizona laws governing the use of autonomous vehicles demand a test driver be behind the wheel to take control in the event of problems or collisions.

Google's Waymo has been one of the most aggressive developers of autonomous car driving technology and services.

The company's robot cars have now driven more than 2.5 million miles on public roads without human help.

The cars have also been involved in 14 collisions while logging those miles.

As well as fitting out existing cars with sensing and navigations systems, Waymo has also developed its own small, two-seater vehicles.

For the Phoenix test, Google will use Chrysler Pacifica minivans.

The news of the test project comes a day after the UK announced plans to get driverless cars tested on public roads and motorways by 2019.

It also comes as the Wall Street Journal revealed Amazon has been working on autonomous car services for more than 12 months.