Well, Steve Levitt has made his reputation on clever use of instrumental variables to avoid the "correlation = causation" trap. So I wouldn't expect him to explain how highway accidents prevent the importation of lemons.

(And Steve Dubner has made his reputation by associating himself as closely as possible with Steve Levitt in an attempt to conceal that Dubner is a mindless knee-jerk contrarian who desperately wants to feel like he's a member of an In The Know elite. But that's not relevant here.)