I saw a new speed restriction on my last trip..

Reduce speed by 20 percent below normal speed limit due to pollution levels. Controlled by radar. So 130kmh becomes
104 kmh : To make it really easy, all in French

Outside Calais / euro tunnell on a bright winter’s Sunday with very little traffic : so no idea where the pollution was coming from!
I did see a French car flashed BEFORE the overhead gantry warning . I was surprised as I didn’t think he was speeding... then the sign !!!! Can’t be sure they were connected. Filters are in fact so good that in certain circumstances, when the ambient air is already polluted, a diesel car will tend to extract more particles from the air than it emits. Emissions Analytics worked with........etc etc

He who dies with the most toys wins...

They do this sometimes in Switzerland - although in my near 15 years here I've only seen it once.

Happens when there's no wind to shift the polution to somewhere else.

They had them around Lyon last winter - but it was fairly clear - they displayed the new limit with some sort of text, but it looked just like a UL variable speed limit.

Belgium reduce from 120kph to 90kph on motorways during periods of high smog.
Depicted by a 90 in a round sign with SMOG underneath it.