[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.