Get this:
some guys at
Los Alamos Laboratories actually ran
simulations on the problem of
gridlock and slowdowns, using
supercomputers to simulate the
traffic flow of
large cities.
It turns out that once traffic reaches a certain density, a slight slowdown of just one or two cars (say, on the freeway to let another car enter in front of them) will propogate backwards causing a big slowdown that just feeds on itself until traffic density decreases sufficiently to allow the jam to unclog.
Since, over time, certain places are likely to see higher traffic density, this also explains why quasi-spontaneous slowdowns occur in the same place over and over.
Interesting, eh?