Sorry to be the skeptic, but (although it's a good idea in theory) I'm not a fan of it:
A) IMHO, more often than not, most ppl on the road aren't paying enough attention while on the road to drive appropriately when there's an emergency vehicle, let alone read a sign on it (esp on a moving emergency vehicle)
B) It's 5 mm LEDs, bulky, & probably comes with a big price tag. Personally, I would prefer something like a rear inner edge, pinnacle, spectralux that will leave a large footprint, because some ppl don't yield for a fire truck any more than they would a wrecker with amber lights. Also, like anything with moving parts, I gotta play devil's advocate and assume that (under Murphy's law) it'll malfunction/won't raise/lower at a key time
C) if I decided to put a message in my rear window while driving code 3, the only useful one I'd think about using is either "BACK THE F@#K UP!" or "STOP RIDING MY A$$!" I'm not saying it would do much, since people ignore the "keep back 200 feet," but oh well
