In the UK we have lots of 'speed warning' signs. When you approach them, if you are exceeding the speed limit, they light up and tell you (and anyone behind you) how fast you are going. And that's all. No penalties. They seem to make a significant difference in residuntial areas. I think they are often paid for by the local community rather than the state.
In Portugal I saw a cute system - if you pass a sensor driving faster than the speed limit, then a traffic signal 200yards/metres down the road turns red for 10 seconds, making you (and again anyone behind you) stop.
The psychology behind these systems is interesting - both rely on shaming you in front of other drivers. The Portugese system goes further and makes other drivers angry with you for speeding.
We have those in the US as well. We use them to see if our speedometers are calibrated as we fly past them.
Roadside technology that *might* work here would be randomly located (and frequently moved) strobes that flash regardless whether there's a camera attached. Every time I see *that* flash (so far I've not been the driver posing for the closeup) I do pay more attention to my speed.