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.