Providing this information makes it easier for people to speed in other places, making the roads a more dangerous place. This rule has definitely cost lives. If you don't think it is unethical, then what is your definition?
Seeing how people react to unmarked speed cameras(police on the side of the road): full brakes with zero concern what is behind them, I'd say not telling people where cameras are is far more dangerous.
That's a meta-analysis that doesn't focus on police activities, but rather unattended camera installations. Results of such studies have hardly been unequivocal, unlike the financial benefits to the camera operators.
Setting aside the fact that you can find a pubmed link to "prove" the efficacy of almost any intervention, the original assertion -- that the roads are more dangerous when drivers are informed of the presence of cameras, police, or both -- is not addressed by this survey.
As speeds have gone up, the death rate has gone down more or less steadily, at least until the last couple of years when drivers collectively decided that their cellphones are more interesting than their driving. A study that purports to exonerate speed traps must first address this rather inconvenient truth.
