Amazon and Google are both tinkering with drone-based delivery systems, but Google is also eyeing a more down-to-earth approach.

As reported by Quartz, a patent awarded to Google this week points to self-driving delivery trucks with compartments that carry individual packages (like an autonomous Amazon locker on wheels).

Here's how it could work, according to the patent: customers' packages are loaded onto a delivery truck's small storage lockers, which can be opened from the outside of the vehicle using a keypad. The "autonomous road vehicle" then drives a specified route to each customer's address. Once it arrives, the customer will get a text message alert with the access code and how long the truck will wait for him or her to open the locker and collect the package.

Dwell periods would be based on "factors such as the value of the package, weather conditions, and the nature of the destination, history of interaction with the addressee, and addressee preferences."

If there are any traffic delays, all customers who have packages on the truck will receive updates alerting them to the estimated arrival time. The patent even suggests that recipients can swipe a credit card to unlock and pay for their purchases on the spot. Presumably, mobile payment systems like Android Pay would also be accepted, but the patent was filed in 2013 before its inception.

The patent is vague on how the self-driving part of the truck would actually work, but given Google's very public research into self-driving cars, we can only assume that the technology would be similar.

It's too early to tell what Google's plans are for this delivery system. The company did not immediately respond to a request for comment. If it does decide to pursue the self-driving delivery truck, the most significant hurdles will be regulatory.

Amazon's drones have already been tested in real-world conditions. They just need final FAA approval. So if Google does plan to launch a ground-based challenge to Amazon's delivery plans, it all comes down to who can convince government regulators to let them fly--or drive--first. And, of course, patents do not always translate into products.

Google, meanwhile, has already experimented with delivery services, albeit with human drivers and mixed results.

This article originally appeared on PCMag.com.