In Russia, it is prohibited to drive a dirty car. I read this on the website of the Dutch motorist association, which is a quite reliable source. It is also reported by BBC News. I do not know the exact wording of the law, so reality may be more subtle.
I've never been to Russia but I believe there's still plenty of long distance dirt roads, in particular in the east such as the Baikal highway between Красноярск (Krasnoyarsk) and Иркутск (Irkutsk)¹:
Or this street in Слюдя́нка (Slyudyanka) in the same region:
With such roads and streets being still quite common in many parts of Russia, I was surprised to see that there is a prohibition against driving a dirty car. Is it enforced at all?
¹I believe this particular road has since been surfaced.