There were a lot of initial economic studies done as electric cars started taking off trying to figure out how to pay for road infrastructure as the USA funded a lot of its infrastructure on gas taxes and that revenue stream was going to go away.

One of those studies found that, if you billed out the infrastructure costs per mile and included all other non-human costs of driving a car around, it was cheaper to have an empty car drive around than to pay for parking in certain urban areas.

Sounds like Waymo figured this out.