So the first signs that I recall popping up use to just measure your speed. This had the unintentional result of people speeding more just to see how high of a speed they could get the sign to register. Typical male mentality. So, the next generation of signs would only flash the speed limit and/or 'slow down'. This newest generation of sign has a LED light bar on the top replicating the ones on top of police cars. We've been conditioned to react to the flashing red and blue of police lights with the instinct to slow down (aside from the criminal element at least...). So, when this sign detects a car speeding excessively, it flashes the red and blue light bar causing the instant "oh crap, there's police, I need to slow down" conditioned reaction. That's my theory at least.
