[Symbiote]

They tie an opaque bag over any incorrect signs, and erect temporary signs (of the same standard, international design) with the new limit. If it's a motorway or similar road, the electronic emergency signs will also show the reduced speed limit [1]. On a motorway, they're often on a gantry, i.e. completely impossible to miss.

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.)