FAR 103.15 states:
No person may operate an ultralight vehicle over any congested area of a city, town, or settlement, or over any open air assembly of persons.
I'm wondering what is meant by "congested area". It sounds like you can fly over a city, town or settlement so long as it's not congested but...how is that defined?
Since it's relevant to this discussion, FAR 103.11(b) also says this in regards to where you can fly an ultralight:
(2) All operations are conducted in uncontrolled airspace.
So you are restricted to Class G airspace, which generally speaking means that major cities and suburban areas are prohibited from being flow over as many of them are under controlled airspace... So I don't think that by "congested" they mean downtown LA or New York, for example. The definition must lie elsewhere...