That's a myth from the past. The Autobahns are usually packed with lorries from eastern Europe, long stretches of construction sites, detours via villages and 50km speed limits to avoid crumbling bridges and of course the everyday traffic with people driving to work.
Germany has been pumping large amounts into extending the Autobahn network in the last 30 years while ignoring rail, so now everyone wants to drive because the train is unreliable, slow and expensive.