Researchers in Japan have recently conducted studies on this phenomenon, known as a shockwave traffic jam.

http://www.youtube.com/watch?v=Suugn-p5C1M

Turns out nothing even needs to happen to cause this sort of jam. Hopefully work like this will help to decongest our roadways.
Loosely paraphrased, since it's been a long while since I've seen the movie:

"The slightest deceleration can cause a ripple effect which persists at the same location for hours." --Ethan Hunt, Mission: Impossible III
"The slightest deceleration can cause a ripple effect which persists at the same location for hours." --Ethan Hunt, Mission: Impossible III

The good news, if you get in an accident due to the slow down and Ethan Hunt happens to be in the area, is that Tom Cruise is the only one who can help you!
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