- A series of 80-60 speed changes on straight road, then just when you are annoyed and don't slow, there is a speed trap.
- Badly marked school zone, I was doing 40km/h already, then a black painted camera hidden in bushes caught me.
It kind of works as deterrent, although I expect that the effect wears off after a while.
This warning activity was tested in court and found to be illegal, as interference with the police undertaking their duties. Their response to the judgement was to switch the warning method to NOT saluting members if they're approaching a speed trap because apparently they couldn't be found culpable for inaction. So they would only salute members if the coast was clear. A bit like a warrant canary.
This probably varies country by country, depending on whether it's a money-making exercise (where the police try to hide) or safety (where cameras are painted bright yellow and the police are clearly visible)
I guess the future of speed traps is "section control", e.g. install cameras at beginning and end of a speed-restricted stretch, and if the time you needed is significantly below the expected one with legal speed, you get a ticket.
But it would be political suicide.
This has been common in Western Europe for decades now.