Ah traffic, a never ending source of wonderment by laymen and scientists alike. I really enjoyed this explanation of the critical mass theory of a traffic jam: A team at Nagoya University in Japan ran a bunch of people around a circle and discovered that 22 cars was the critical number. Once you had that many on the track small changes, like a split-second braking, reverberated through the system.
Jonah Lehrer beautifully explains this in terms of phase transition:
This is actually a pretty familiar scenario for particle physicists, who are used to studying phase transitions, such as the transformation of liquid water into solid ice. In this case, the critical threshold is temperature, which triggers clusters of molecules to slow down and form a crystal lattice, which then spreads to nearby molecules. A traffic jam is simply a solid made up of idling cars.