Researchers in Japan have created a live model of the so-called "shockwave" theory to explain traffic congestion.
"The mathematical theory behind these so-called 'shockwave' jams was developed more than 15 years ago using models that show jams appear from nowhere on roads carrying their maximum capacity of free-flowing traffic – typically triggered by a single driver slowing down."
"After that first vehicle brakes, the driver behind must also slow, and a shockwave jam of bunching cars appears, travelling backwards through the traffic."
"The theory has frequently been modelled in computer simulations, and seems to fit with observations of real traffic, but has never been recreated experimentally until now."
See the video linked on the source article page.