Complex sentence: German autobahns have no federally mandated speed limit,--although limits are posted (and enforced) in areas that are urbanized, substandard, accident-prone, or under construction.
Split into simpler sentences: German autobahns have no federally mandated speed limit for some classes of vehicles. However limits are posted (and enforced) in areas that are urbanized, substandard, accident-prone, or under construction.