Waymo has partnered with UPS to carry packages around Phoenix, Arizona using its self-driving cars.

Although not yet delivering to houses, the cars will transport items from UPS stores to the courier's processing hub in Tempe.

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The cars will drive themselves, but a Waymo safety driver will sit behind the wheel to monitor the vehicle and take over if it encounters an unfamiliar situation that it can't work out for itself.

The pilot service will begin this quarter and is described by Waymo as a "long-term plan," so it would be fair to assume UPS will use Waymo vehicles more in the future.

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Waymo says the trial will "explore customer and operational benefits and illustrate how the Waymo Driver can deliver on behalf of clients safely, efficiently, and eventually, at scale."

Waymo Driver is what the company calls its autonomous technology, which, in theory, can be fitted to a wide range of vehicles. This will be key to the UPS partnership's success as, while the Pacifica is a fairly large and spacious vehicle, it is primarily designed to carry people and not parcels. If the Waymo Driver can be fitted to a delivery van, or even a purpose-built autonomous truck, then the efficiency of the delivery service will increase.

Waymo Chrysler Pacifica Waymo

On that note, Waymo is currently testing its self-driving technology with trucks, also being driven on public roads in Phoenix.

The use of Waymo cars will help UPS get packages "...to our sortation facilities sooner and more frequently, while also creating an opportunity for later drop-offs for next-day service," said UPS, in a statement.

UPS raises an interesting point here: Once the cars no longer need a safety driver behind the wheel, they will work around the clock, transporting parcels outside of current delivery drivers' shift patterns.

This trial aims to move packages more frequently, rather than shifting a huge amount all at once. That way, valuable but limited space is freed up in UPS' retail stores throughout the day rather than with a single large collection.

UPS also announced this week that it has ordered 10,000 electric delivery trucks from Arrival, a UK startup.