phantom jam (plural phantom jams)

1. (automotive) A slowdown in traffic that occurs without an apparent reason.
   • 1996, Achim Bachem, Dietrich E Wolf, Michael Schreckenberg (editors), Traffic And Granular Flow, page 76:

     Eventually, the phantom jam always dissolves. Sometimes the phantom jams are large and sometimes they are small.

   • 2001 August 25, Sherman Cahal, “ARTICLE: Traffic: Stuck in the Maddening Crowd”, in ‎^[1] (Usenet):

     The leftover phantom jam might be miles away from its orgin.

   • 2003, Elsevier Limited (publisher), Control in Transportation Systems ... a Proceedings Volume from the IFAC Symposium, page 335:

     One of the most affected problems is the behavior of freeway traffic, which includes the appearance of "the phantom jam."

   • 2012, Martin Treiber, Arne Kesting, Traffic Flow Dynamics, Data, Models and Simulation, page 569:

     Finally, when interpreting accidents as events “out of thin air”, there are, in fact, phantom jams.

2. (automotive) A traffic jam that appears at a distance on a highway, but in reality is an optical illusion.