Maybe correlate it with central database for sanity. Preferably daily updated git.
I think I read somewhere that it's someone's job to make regular checks that the temporary signs (and covers) are still correct -- they are an important part of the worker safety requirement for the construction crew.
When a speed limit changes, the rule is for the sign to be shown on both sides of the road. There are then repeating signs for the current limit at some regular interval.
The UK is dense enough that having expensive electronic signs on all motorways isn't an unreasonable cost; I understand that's not practical in the USA or Australia.
[1a] https://i2-prod.birminghammail.co.uk/news/midlands-news/arti...
[1b] https://www.ageas.co.uk/globalassets/solved/30072018_road-sa... (possibly this style is no longer used, I drive very rarely in the UK so I'm not sure.)
I've had speeding tickets forgiven in Australia because the sign simply wasn't visible enough due to overgrown trees and a 2 lane off ramp separating the sign from the road it applied to, let alone the sign missing altogether.