I could not find a definite definition of congested area .
As mentioned here : [...]
neither the FAA nor the NTSB has ever provided [...] a precise definition of [...] a "congested area."
Rather, a "congested area" is determined on a case-by-case basis.
According to the Board, "the determination must take into consideration all circumstances, not only the size of an area and the number of homes or structures, but, for example, whether the buildings are occupied or people are otherwise present, such as on roads."
The same is echoed here : [For the definition of congested area,] FAA and the NTSB have opted for taking a "case-by-case" approach in determining how to apply certain terms.
AOPA states that : Congested areas .
"Over any congested area of a city, town, or settlement, or over any open air assembly of persons, an altitude of 1,000 feet above the highest obstacle within a horizontal radius of 2,000 feet of the aircraft." The FAA does not define congested area in the FARs or in the Aeronautical Information Manual.
Interpretations in low-flight enforcement cases are not consistent for purposes of drafting a precise definition.
Such a determination is usually decided on a case-by-case basis, and in the cases that we've seen, congested has been interpreted rather broadly.
For example, a highway with moderate traffic was found to be "congested," as was a seaside area where 200 to 300 persons were sitting on the beach or bathing in the water.
The same can be seen here , and here , and here .
aviation.stackexchange.com/users/65/Farhan
It is not explicitly defined in the FAR Definitions and it seems the explicitly do not define it.
I would interpret ( please keep in mind
this is my interpretation and not law )
a congested area/assembly of people, as anywhere you could not safely land the aircraft in the event of an emergency without endangering others and or yourself.
This may better by explained with an example.
Lets say you are flying low over main street at rush hour, your engine goes and you are forced to make an emergency landing.
You have limited distance and direction to go and can not safely land the aircraft with out hitting either a building, people or vehicles on the road thus endangering them.
With that being said, you are correct that class B generally lies around the big cities in this country, and other cities may reside in class C, you can still have a congested area or open assembly of people (concert, state fare etc) in class G airspace.
aviation.stackexchange.com/users/3677/Dave
If a regulatory authority does not define the word, then by default it is the common definition of the word.
Un-congested vs congested.
Think of the phrase Open space and occupied space.
If flying over occupied homes or businesses it is by definition congested.
If you are flying over open land it is un-congested.
In reality, the FAA uses many words and phrases with out providing what their definition is and leaves it to the common understanding rather than a unique definition.
Simple English...but not to those who push the limits.
aviation.stackexchange.com/users/34117/Randy D Andrews
While getting my Aerospace Engineering degree a "Regulations and Legal Interpretations" class was mandatory.
As part of that we studied cases against pilots such as flying low over farm houses.
The judges have been persuaded to accept 2 or more people as "Congested" or an "assembly" based on existing automobile traffic laws that fines people for violating speed limits for crowds or people (plural).
For example, "when children (pl 2 or more) are present" .
Take this at face value.
I don't agree with it
but it seems to be a case where the Government feels more successful by being vague then being reasonable.
To me something like "more than 25 people in a 1 mile radius" would be more quantifiable.
aviation.stackexchange.com/users/25735/jwzumwalt
You can fly over a city which is congested, by following railroad tracks, etc, those areas are uncontested.
But, you must be in class E or G airspace, unless you have clearance from the controlling agency.
The FAR 103.11(b) is only for flying with strobes, during civil twilight.
You must be in uncontrolled airspace, which is class G, generally, but not always, starting at 700 or 1200 feet AGL.
Where I live ..0
-500 feet is class G and class B starts at 500' ago.
So I could fly my ultralight at 400', 20 minutes after sunset, along railroad tracks thru a major city.
A bad decision is called pilot error, your job is to keep others safe from your stupidity. :)
aviation.stackexchange.com/users/35094/William
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?
What is meant by a "congested area of a city/town/settlement" in FAR 103?
aviation.stackexchange.com
<faa-regulations><faa><ultralight>
aviation.stackexchange.com/questions/15611
aviation.stackexchange.com/users/511/Jay Carr
Congested area is not defined in the FAR Definitions. Interpretation of a "congested area" is subjective. There is no explicit definition. 
Interpretation of a "congested area" is subjective. There is no explicit definition. 