Add MAC addresses passing through major intersections and you've got some quality data to work with.

California is already rolling out "smart" licence plates, so not far off.

What do you mean by "committing license plate numbers to a blockchain"?

Probably that the records are preserved in some way that they can't be redacted or removed later.

What records? I don't understand what this is supposed to describe. Just writing every plate number some device has seen to the append-only DB? Is that it? What's value in that?

I think the value would be with recording a location and time with the plate number, not just that a plate exists.