Last fall the folks at NYC's Transportation Alternatives produced a lovely video revisiting the seminal study by UC Berkeley professor Donald Appleyard on the connection between automobile traffic and a neighborhood's social health.
In his groundbreaking book, Livable Streets, Appleyard studied three San Francisco residental streets of similar dimensions. The main difference was their level of traffic. Appleyard surveyed residents to understand their sense of their neighborhood and their connection with others.
Between the low traffic and high traffic street, neighbors' perceptions were strikingly different. On the low traffic street, residents identified three friends on the street compared to .9 per street on the highly trafficked one. Why? In part, people on low traffic streets tended to spend more time on the sidewalk and street, creating more time for interactions with neighbors.