  While the impact of crowding on the diffusive transport of molecules within a
cell is widely studied in biology, it has thus far been neglected in traffic
systems where bulk behavior is the main concern. Here, we study the effects of
crowding due to car density and driving fluctuations on the transport of
vehicles. Using a microscopic model for traffic, we found that crowding can
push car movement from a superballistic down to a subdiffusive state. The
transition is also associated with a change in the shape of the probability
distribution of positions from negatively-skewed normal to an exponential
distribution. Moreover, crowding broadens the distribution of cars' trap times
and cluster sizes. At steady state, the subdiffusive state persists only when
there is a large variability in car speeds. We further relate our work to prior
findings from random walk models of transport in cellular systems.
