Do the Amish count? Various retro-communities?
Keep in mind that Central Venice is car-free (although there are cars in the suburbs). My prediction is that a new city will spring up in the Florida Keys named Atlantis, modelled after Venice with no cars. Perhaps with Dutch engineering for water control.
Of course, in a few decades, it is quite possible that New Orleans will also look like Venice.
The USA could at some point annex one of the Caribbean Island nations. Haiti and Cuba are among the lowest in the world for car ownership. Although I imagine that the 51[SUP]st[/SUP] state will be Puerto Rico which isn't too different from the US average. The Caribbean governments like their independence, but both the USA and Cuba would benefit significantly with a merger, if the two governments would ever put aside their petty differences.
New York and Manhattan Island have been investing in car-free infrastructure for decades, so they may well lead the nation if there was to be a shift to a car-free structure. I could imagine NYC as one big walking/biking mall by day, and trucks by night.