Any big town in America is going to be ruder than rural areas simply due to immigration from a variety of cultures, many of which aren't all that polite, civilized, educated, or even well-intentioned. Others are introverted or guarded being in a new culture, or uneasy with freedom now that they have it but don't really understand it. It will only get worse, we will never co-exist in close quarters as tribalism is an instinct necessary for survival of the species. The whole immigration schtick is all about creating a bland, unemotional slave race to serve the Buffetts, Kochs, Bloombergs and Gates' of the world. It serves no legitimate humanitarian goal and those who encourage it are no better than pimps.
Oops, slid into a mini-rant there.
The rudest Americans are New Yorkers. We have encountered extremely rude New Yorkers in Oregon, Hawaii, California, Montana, Nevada and Florida.
The rudest "culture?" I've encountered are the Chinese masses who've over-run Vancouver, BC the last few decades. They will knock down old ladies to cut in line shopping or for a seat on the bus. No exaggerration.
End thought: Amount of Personal space = Chance of Politeness Encounter