Image copyright Waymo Image caption Waymo's cars will deliver Walmart customers to their groceries

Google's sister-company Waymo has announced a trial in which its self-driving cars will ferry shoppers to and from a nearby Walmart store to pick up their groceries.

For now, the pilot is being restricted to 400-plus members of its early rider programme in Phoenix, Arizona.

However, it indicates how the tech giant thinks the autonomous vehicles could be deployed if and when they exit the experimental stage.

One expert said cost would be key.

The only word on pricing so far is a promise to offer participants discounts when they order goods via Walmart's Online Grocery Pickup service as part of the deal.

"If this is rolled out properly you would expect there to be a reasonably high threshold in terms of the price and spend commitment to justify the service," commented Julie Palmer, a retail expert at the consultancy Begbies Traynor.

"You'd expect it to be limited to shoppers buying higher value items."

Google v Amazon

The Walmart partnership had been rumoured after shoppers spotted Waymo-marked parking spots outside the store involved earlier this week, and shared the discovery on Reddit's Self-driving Cars forum .

Image copyright Walmart Image caption Eagle-eyed shoppers spotted the Waymo parking spaces in advance of the announcement

Some suggested that the tie-up could offer an alternative to Amazon's Prime Now quick-delivery service.

"Google and Amazon are both looking at cutting-edge ways to enter the food retail market, and we're only beginning to see them flex their muscles," remarked Ms Palmer.

The announcements comes two days after Waymo's chief executive tweeted that its cars had driven a total of eight million miles on public roads, which is double what its tally had been in November.

Image copyright Waymo Image caption Waymo launched its test programme in Phoenix in June 2017

"This real-world experience, plus over five billion miles in simulation, is how we're building the world's most experienced driver," John Krafcik added .

Self-driving rivals have also had announcements of their own this week: