You'd need the equivalent of a 228-lane Brooklyn Bridge to move all those people into Manhattan during Monday morning rush hour.
At best, it would take 167 inbound lanes, or 84 copies of the Queens Midtown Tunnel, to carry what the NYC Subway carries over 22 inbound tracks through 12 tunnels and 2 (partial) bridges. At worst, 200 new copies of 5th Avenue. Somewhere in the middle would be 67 West Side Highways or 76 Brooklyn Bridges. And this neglects the Long Island Railroad, Metro North, NJ Transit, and PATH systems entirely.
Kinda puts the subway in perspective, doesn't it? And don't miss the map at the bottom that shows the size of the parking lots needed for all those cars.
Mike Frumin took the NYC subway ridership data from all the way back to 1905 (!!) and graphed it on a map, with a sparkline of the ridership data for each stop. Frumin explains the project a little more here.
The result, after much whacking, is, I think, compelling, but you'll have to see for yourself. The general idea it that the history of subway ridership tells a story about the history of a neighborhood that is much richer than the overall trend. An example, below, shows the wild comeback of inner Williamsburg, and how the growth decays at each successive stop away from Manhattan on the L train.
Update: Here's another representation of the same data. In this one, the ridership numbers are represented by the thickness of the subway line.
Michael Frumin tried to get some NYC subway data from the New York City Transit Authority through Freedom Of Information Legislation for a project he wanted to do, but they denied his requests. "Given a database of anonymized Metrocard 'swipes' over some small period of time, Frumin imagined that a multitude of explorations could be embarked upon. Below is a concept sketch for one specific project idea -- a visualization, for each station in the system, of the range of locations in the city that people travel to from that area." Nice Minard-esque prototype map.
Eyebeam's Mike Frumin has released OGLE (OpenGL Extrator), a software package for extracting 3-D data from Windows applications. This means you can do stuff like grab the 3-D likeness of your World of Warcraft character and print it out on a 3-D printer or insert him into a Manhattan landscape (grabbed from Google Earth). Announcement here.