Sha Hwang, from Stamen Design, has recently created a series of though-provoking visualizations on the NYC subway ridership over the last decades. Using a spreadsheet with the annual recorded entries at each station in the NYC subway system, Sha Hwang decided to plot them on an interactive map (using modest maps) applying two different methods: scaled dots (centered on each station) and lines (unifying the different stations). Even though there are some pertinent questions, in the original blog post, as to the danger of extrapolating traffic flow from individual station boarding, this initial experiment is notwithstanding very valuable, since it could help explain many urban and sociological changes in the city over time.
Image one shows ridership in 2006 using the lines method and showing the time slider on the lower right. The second image illustrates the ridership difference - highly noticeable in the mid-town area - between 1977 (left) and 2006 (right).