Road grime can be insidious, especially if you are a hitchhiker, (especially especially if you are a long distance hitchhiker), or a hobo, or a nomad, or a pilgrim. Fortunately, people have set up roadside laundries to take care of that grime.
Currently, they are spaced about 300 miles apart from each other on a grid system, so you build up a lot of grime traveling between them. Their locations are indicated by hobo hieroglyphics scattered along popular migration routes.
They aren’t very big and actually don’t do a very good job getting the grime out, but at least the grime is clean.