A reader writes:

Yeah, about those anti-rubbernecker screens. Tom Vanderbilt discusses them in his surprisingly fascinating book Traffic (quick summary: we're idiots and that is reflected in our driving). The question you pose is indeed the case: people just look at the screens, causing the same traffic problems. (The relevant bit is on this page of the book.)

The screens will work for things like construction, where the same screens are in the same place every day; we get used to them. But they are pretty useless for dynamic things like crashes. Oh, and the vehicle delivering the screens usually gets stuck in the traffic caused by the crash. Flawless plan!