If you're interested in the physics of it, there's a PDE that explains these backwards propagating density shockwaves (aka traffic jams). http://en.wikipedia.o

If you're interested in the physics of it, there's a PDE that explains these backwards propagating density shockwaves (aka traffic jams). http://en.wikipedia.org/wiki/Burgers'_equation http://en.wikipedia.org/wiki/Burgers'_equation