Building a really good mass transit system is terrifyingly expensive. And yet, cities that have such systems can derive huge benefits from them. The construction of Metro laid the groundwork for Washington, DC's current renaissance and cities like New York and London continue to reap enormous benefits from transit infrastructure investments that were initiated far, far in the past. Politicians, however, don't get appointed to 50 year terms that allow them to take credit for the long-term benefits of their decision to spend big money up front in the short run.

Consequently, there's been a lot of discussion recently in the political world of tweaking the way we finance infrastructure so as to make long-term investments more viable in budgetary terms. Rob Goodspeed has a good rundown of the current state of play.