This is interesting, half of all Americans live in 146 of 3,000 counties in the US.
Using Census data, we’ve figured out that half of the United States population is clustered in just the 146 biggest counties out of over 3000. [icopyright one button toolbar]
Here’s the map, with said counties shaded in. Below the map is the list of all the counties, so you can see if you live in one of them.
This may explain much of the political polarization in the US – the urban VS rural divide. I think this actually underestimates the demography. Although here in the Portland Oregon area Multnomah county is included the urban area consists of 4 counties where most of the states population lives.