While we're on the subject of maps, I just ran across these terrific maps of subway systems in different world cities, all drawn to the same scale.
When they're all mapped out this way, you really get a sense of how the different systems serve different needs. Some, like Paris' Metro, are compact and tight, and they're clearly designed to help move people through the city:
In contrast, a system like Washington, D.C.'s Metro is sparse and spread out, and intended to be more of a hybrid commuter line:
(via Matthew Yglesias )