I think it varies regionally. Some forces have a very good attitude to policing in the public interest and will make sure all the markings are visible. Those forces will also not 'hide' their vans needlessly.
I see it in many ways - if it's a difficult road where speed could be a cause of needless RTCs then surely drivers who are astute would be aware of the difficult road and not be driving excessively quickly !! If you don't know the road at all then it's even easier to apply this thought. If you do know it then you know the pitfalls - that's a no winner sadly.
There are also some places where the posted limit is needlessly low and is sometimes over enforced. Again, this is just HDi's opinion.
Other situations are where the posted limit (though being marked in accordance with the RTA) is is not marked sufficiently.
If speed limits were applied across the country with the ethos being to let drivers know the TYPE of hazard they are most likely to encounter, coupled with sensible use of speed camera vans then things would be better.
Provided, of course, that drivers use their own eyes and brains too.
There is, however, one perfect way to avoid detection or penalty. Know the limit and don't exceed it ! ! ! ! This however, is not an excuse to drive badly 0.000001mph below the speed limit in force.
In my estimation Traffic Police are far more sensible WRT to 'petty' speeding than any camera will ever be. Those guys do a damned good job and it's a difficult one. They have to pick up bits of limbs strewn across the carriageway after a major smash. No wonder they don't like moronic driving, whether or not above the speed limit.