Good question. The study by the DIW was broken by FT Germany and some quotes found their way to other news outlets, but I can't find the original study so I can

Good question. The study by the DIW was broken by FT Germany and some quotes found their way to other news outlets, but I can't find the original study so I can't comment much on their methodology. What I can say is that at least there are different usage and resulting deterioration patterns for the different types of roads, for which statistics exist. Also, for the autobahn there exists an additional source of income, which is tolls for trucks above 7.5 tons.