This thought has been percolating for some time but these two posts by Matt Yglesias yesterday really solidified it for me.

Essentially, America should always be building lots and lots of stuff. What that stuff is, mind you, is up for debate. I am open to ideas that, during times of strong economic growth and low unemployment, any attempt by the public to build stuff should be met with some fundamental level of cost-benefit skepticism since there are significant opportunity costs to that building – ie, you are diverting resources away from the theoretically-more-efficient-private-sector building of stuff. But when the economy is weak and unemployment is high, this is excatly the time we should be saying "all these people who were building stuff naught but a year or two ago are now sitting idle, and the public would benefit from building lots of stuff!" and we should then proceed to mobilize the workforce in building new transit projects and the like. Unlike boom times, where the private sector may be building new housing and infrastucture, there is no opportunity cost to building all this stuff! It is actually astoundingly misguided to say "since the economy is bad we can’t afford to build stuff." This is part of the broader reification of money that has a zombie death-grip on our economic thinking.