[drekembemutombo]

I've heard somewhere that the speed displays actually help a lot with slowing down the traffic where they're put up. I slow down when I see them too, not because I'm afraid of enforcement action, but because I just forget how fast I'm going sometimes and that board reminds me. Plus, it feels kind of satisfying when you do slow down and you get the green smiley face along with a message thanking you.

[notacoward]

I think there's a threshold effect here. People will slow down a bit but not too much. Above that threshold, the reward of being recognized as law-abiding is outweighed by the inconvenience/annoyance of being coerced to what seems like an unnecessarily low speed. Similarly, if the signs are everywhere people start to tune them all out. While they're very cost-effective in general, it's definitely possible for a town to overplay their hand and ruin their effectiveness.