gotta remember that one
The thing is, even if they're not working, the road is often "half done" with uneven surfaces and you have to wind thru traffic cones and stuff... hence the need to leave it 40km.
Down here if they feel it's safe to do the normal speed when the workers aren't around, they put a cover on the 40 sign.
It's really not rocket surgery... if you see a sign, just do whatever it says.