We've got those everywhere. Everybody ignores them. They're probably one of those many things that work while they're new, but stop working after people acclimatize.

They work on me and drivers around me. I regularly see people dropping their speed and being more mindful on roads with them.

The trick is to occasionally (on an unpredictable schedule) put a police cruiser in the same vicinity if the average speed creeps up too much. This turns the readout into a legitimate warning. When you put them out without enforcement then everyone learns to ignore them.

Police regularly giving speeding tickets lowers road speeds. If the police regularly hang out at speed indicators, then people will learn that and only slow down at the indicators. OTOH, with regular enforcement and without the speed indicators people will slow down everywhere. I'd rather have people slow down everywhere.

[delayed]