In our area cars are more or less a necessity. Public transportation is sufficient but it limits convenience significantly (though we and others have lobbied for defined improvements). However many people we know are one-car households where their parents would have likely been, or are, two-car households. Additionally economics has played a clear role in our social group. We know several people who have traded to hybrids, sold their second vehicle, purchased a less expensive model than their previous car, or in more than a couple cases changed employment/arranged to work from home or moved downtown and forgone cars altogether.
Specifically to us we’ll probably always have a car or, more likely, a truck because of the convenience and our “local” travel style. But like some of our friends, if we moved into the city it would be hard to justify owning a car and would be significantly cheaper just to rent as needed.
As far as the US, I’ll believe we’ve reached the peak when the same declining trends are evident in a booming economy. As far as the world, urbanization can cause a real drag, particularly when urban areas are poorly planned/maintained, but I’d say car sales/ownership are likely to increase as china, india, africa, brazil become richer. I don’t know enough about the ratios and percentages to say if it will be a net increase or not but hopefully they will decide with an approach of greater moderation and less “status symbol”.