From a reader named Stephane:
Very recently I drove through a couple of small villages in the northwestern part of Belgium (near the border with France). A couple of road signs caught my attention. When you reach a village there's a sign (in Dutch) saying "here, X percent of the drivers stay within the speed limit." Then when you reach the next village there's the same sign except that the percentage is different. Usually it's around 90% (87% in one village, 91% in another, etc.).
I don't know how they collect the data or even if the numbers are real. I also wish I knew the trends, how often they change the signs, how many villages participate in this safety initiative, etc. Then I wondered: where does this idea come from? Have you heard of anything like this before? If yes, is this effective to slow cars down?