From the article on Information Aesthetics:

In a groundbreaking study conducted in 1969, Donald Appleyard, who was a professor of Urban Design at the University of California, Berkeley, provided the first empirical evidence of the impact of traffic on neighbourhood streets.

In particular, he investigated 3 different streets in San Francisco that were chosen to be as identical as possible in every dimension except for one -- the amount of traffic on each street. The study was able to show that just the mere presence of cars, with their implied aspects of danger, noise and pollution, crushes the quality of life in neighbourhoods.

Revisiting Donald Appleyard's Livable Streets from Streetfilms on Vimeo

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