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It’s the former. In many cities cars are required to have a badge in their windshield signifying / signaling (?) how polluting (is that a word?) their car is. From red (really old and bad) to green. This applies to both gasoline and diesel cars and e.g. really dirty diesel cars (euro 1 and 2 norm) are completely forbidden (red badge).
Then additionally some cities have streets banning diesel cars completly or requiring some very strict exhaust norm (euro 6? Euro 7; I don’t know).
How is this enforced. E.g. by checking parking cars for their badge or by selectively stopping cars (and be it for other violations, yes). There obviously are some fines. That’s it I think and it mostly works (I guess, not living in a city myself).
My t440s is 10 years now. Still going strong as my main and only computer. Although, those frame.works’s look really nice.