  We scrutinize the temporally-resolved speed of active cargo transport in
living cells, and show intermittent bursting motions. These nonlinear
fluctuations follow a scaling law over several decades of time and space, the
statistical regularities displaying a time-averaged shape that we interpret to
reflect stress buildup followed by rapid release. The power law of scaling is
the same as seen in driven jammed colloids, granular, and magnetic systems. The
implied regulation of active transport with environmental obstruction extends
the classical notion of molecular crowding.
