It takes time to do so. This is no different from a queue of cars at a red traffic signal, which does not dissipate immediately when the light turns green. In both cases, freeways and streets, jams need time to dissipate, and the heavier traffic is the longer the process. During the rush hour a freeway jam can take much longer to dissipate than to grow in the first place. The evolving jam moves over the freeway in the direction opposite to traffic, affecting it far from the location of the original cause.
Drivers exiting the jam see nothing unusual, much to their surprise.
Note the brief duration of the incident and its lasting effects. A general purpose simulation package for PCs or Macs and its user manual can be downloaded from the Publications Page. The cell transmission model.