Here’s an interesting new working paper: Subways, Strikes, and Slowdowns. Unfortunately, there doesn’t seem to be an ungated version. But here’s the summary: the author argues that mass transit has a significant impact in reducing traffic congestion, even when it carries only a small fraction of commuters. Why? Because commuters who take mass transit are, very disproportionately, people who would otherwise be driving on the most congested routes. So even the small number of people taken off the roads has a surprisingly large effect in reducing travel delays.
But Krugman admits to sinister motives behind his views of mass transit. Okay, not really. He's just making fun of George Will.