Road Signs that are ignored

[Foss62]

On the A11 travelling south, just east of Cambridge there have been temporary speed limit signs for many months (maybe even years?) that are now completely ignored by 95% of drivers.
The reason for the reduction (which is 40 at its lowest) is that there is a problem involving the central barrier but, strangely, the opposite carriageway is unchanged.
I think regular users of the road see no roadworks, obvious problems or speed enforcement, so, long ago, gave up taking any notice of the signage.
I tend to stick more or less to the posted limits along with a very few others, convincing myself that one day there might be a camera van and then I wouldn’t look so stupid (but those caught 80/40 might be struggling a bit).
What do others here do in similar situations?
Incidentally I think there is a wider point here that this sort of thing erodes respect for signage in general.

[Foss62]

Pica-Pica said:

I have seen a few recently.
I believe they are mostly for central barriers, awaiting repairs, that will not perform to their original intent, and thus will not offer their full protection if struck. So that makes sense. Funnily enough, my wife said that these reduced limits should explain why, and then the next minute we come across a 40mph limit with the central barrier comment.

Although the reasoning seems sound, subsequent treatment of these stretches (which as C.A.R. confirms can be there for two or more years) does not seem to be well thought out.
The A11 stretch mentioned is part of a major route, so presumably the local constabulary, highways authority etc. are well aware that the limits are ignored by the regular users. They must also know that visitors to the area and people who routinely obey any posted limit are likely to cause huge speed differentials.
If the risk (caused by the barrier) is high then they should surely enforce the speed limit. If not then it would be sensible to remove the speed limit to avoid the created risk.