one-car traffic jam 

(noun) a vehicle which you are behind is being driven in such a manner that would make you think you are in a traffic jam but for which there is no other cars in front of it; a car being driven unnecessarily or deliberately slow to piss off people and create a traffic jam; a potential road rage victim; driving so slow as to delay everyone behind the vehicle but also ironically delaying the driver of said slow vehicle.

Not always but often on a city street where passing is neither possible or legal, and on highways or freeways during peak periods, occupying lanes in such a manner as to create congestion.

Steve: "Is traffic really bad on this street for some reason?"
Mike: "No, you're behind a one-car traffic jam."

Marco: "Is there an accident ahead? Why is this guy in the Honda driving so slow in the passing lane?"
Eric: "No accidents that I can see. That guy is a one-car traffic jam."

one-car traffic jam by ShoulderCheck2005 March 11, 2014