And yet roads are full of highly visible warning signs whereverr there is a turn that might not be safe at slightly beyond general maximum speed in some weather conditions.

I'm not sure I've seen weather condition specific warning signs, apart from the speed limits being lowered for the winter season by swapping the signs themselves.

Can you link to an example?

They're pretty common in some places. The E17/A22 entering France for example has these overhead signs[0] that go from 110 in good conditions down to 70 in very bad conditions or even 50 if there's congestion. There are also similar signs in Belgium but they're more rarely adjusted depending on weather, it's usually just according to traffic.

[0] https://maps.app.goo.gl/pbCArbN5yraJZFBw9

I can’t seem to find a link, but I know there is a fog-prone Interstate zone in eastern Tennessee (probably I-81, might be I-40) with variable speed limit signs not terribly different from those on UK smart motorways.

Hungary has an elegant, low-tech solution. Broad white stripes are painted ~100 m apart on the shoulder of the road. Fixed signs indicate the allowed speed - if you can see three, it’s 130, two is 110, one is 80 (or something like that - it’s been a few years).

I wasn't even referring to signs that are only applicable in specific weather conditions, just pointing out the absurd amount of handholding drivers tend to get. Some drunk chump might run straight into the trees? Add more reflective chevrons. Someone still crashed? Add flashing LEDs.

And yes, German signage certainly does weather dependency, speed limits with "Zusatzzeichen 1052-36" are quite frequent (I think it's put up everywhere the surface doesn't drain as good as it should, e.g. if surface angle changes slightly from the ground setting after construction)

The UK's "smart motorways" project does this. https://en.wikipedia.org/wiki/Smart_motorway

Actually turns out they're a terrible idea, tbh.