Pressure Waves happen when people stamp on their brake pedals. The breaking causes people behind to break, and so on in a chain reaction or domino effect. See windscreen washer domino effect. The effect has been studied by physicists, who liken the effect to pressure pulses in a liquid. Observations have shown that these waves of braking travel back for miles along motorways (freeways). Drivers will experience a traffic jam for no discernable reason, which will end just as quickly as it seemed to start.
The difference is, though, that when in a confined space, under pressure liquid will move faster. Traffic, on the other hand, slows down. Does this mean that it is viscous? Automated Traffic Control Systems might be able to make the traffic work more like a fluid, and thereby reduce traffic jams?
Perhaps the induced bahviour of a passing pressure wave is analogous to a meme passing though a group of people? | 
