Farmland + rain + tractors riding through fields and then pulling out onto roads = mud, and lots of it.
In our walking routine, Dobby and I have our habits. If the weather has been reasonably dry, we will walk the unpaved roads between fields behind our house. If it has been raining, we avoid the puddles and mud and stick to the paved streets.
These days though, it does not matter, there is dirt and mud everywhere. Farm equipment moves from field to road and road to field throughout the area and throughout the day. Anything - people, animals, cars - that moves on the roads gets dirty. Both Dobby and I have to wipe all the mud off of our feet and legs when we return from our walk now.
The other day as I was driving into Mons, I noticed a van with the word "sale" written in the grime. At first I thought, "it's for sale" then I realized nope "it's dirty." The French word for dirty is "sale" and I guess the practice of writing in a car's grime transcends nationality.