The honest answer would be "it depends where they are"
Average speed limit SPECS along road works are obviously a good safety device, although if the road conditions allow (ie only one lane closed, no traffic cone slaloms) the should be turned off when the workers aren't there.
Many cameras are placed where they should, but there many which are hidden/obvuscated/on straight flat roads simply designed to make money.
---------- Post added at 21:42 ---------- Previous post was at 21:37 ----------
What I mean is if they're placed somewhere and are bringing in a nice income, showing they're not deterring speeders, then that is a good signal that the road is still going to produce accidents, and as such, other measures should be taken to reduce them.
Something like rumble strips, or road narrowing markings, or straighten the nasty bend, or put up a barrier, chop down that tree etc
It is not acceptable for the "safety partnerships" to sit back and make money while road safety hasn't been increased, if you see what I mean.
Have more ANPR cameras instead