Not every plate may idenfity a person (cars can be owned by companies and used by different employees and so on), but a significant number of cars is owned and driven by a single private owner. You can't easily discern if a license plate does or does not map to one individual, thus if you can't prove that the license plate does not map to a specific person, you should assume it does.
Thus the license plates can easily be argued they are PII.
Thus the license plates can easily be argued they are PII.