phantom traffic jam
n.— «We’ve all been there—stuck in traffic, inching along, running late and getting angry when suddenly everyone starts moving. Just like that, the road clears. No flashing lights, no mangled cars, no clue to suggest what went wrong. They’re called phantom traffic jams.» —“MIT Hopes to Exorcise ’Phantom’ Traffic Jams” by Tony Borroz Wired: Autopia June 17, 2009. (source: Double-Tongued Dictionary)