Mike Frumin at frumination created this map to illustrate what NYC would look like if there were no NYC subway. Specifically, it would need a lot of parking spaces. He recently wondered, after receiving subway passenger count data, " What would it take in terms of auto facilities to replace the morning rush hour carrying capacity of the NYC subway?"
Using data from 2007 passenger counts, he found that "From 8:00AM to 8:59 AM on an average Fall day in 2007 the NYC Subway carried 388,802 passengers into the CBD on 370 trains over 22 tracks. In other words, a train carrying 1,050 people crossed into the CBD every 6 seconds"—whereas "Over this same period, the average number of passengers in a vehicle crossing any of the East River crossings was 1.20. This means that, lacking the subway, we would need to move 324,000 additional vehicles into the CBD (never mind where they would all park)."
So, if there were no subways, Frumin estimates, "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." More data here. [via Kottke]