In 1981, Donald Appleyard published a study involving three different streets in San Francisco to gauge the impact cars have on a street's community atmosphere. The study, "Livable Streets", has now been made into a video as part of Streetsblog's Streetfilms series. The series has other videos which are worth checking out.
I'd be interested in seeing how the study scales to different kinds of neighborhoods and in cities not neccesarily as dense as San Francisco. And at what density does the community start to fall apart? In the suburbs, for example, traffic is almost non-existent but so is a community feel. Something planners used to be very adept at was designing livable communities with guaranteed results, but today that delicate balance has been lost.