Why do Americans think "traffic" means congestion? Traffic is the people on the road. You can say things like "let me check the traffic" or "the traffic is really bad", but if you use the road you are traffic. It's nothing to do with congestion, if you use the road you are traffic. It's this kind of subtle language mistake that reveals a whole country full of main characters: I'm the one travelling, everyone else is just there for my inconvenience.
