The Mathematical Society of Traffic Flow in Japan used a test track to replicate traffic jams that seem to occur for no reason. These "shockwave" traffic jams travel backwards at about 20 km (~12 mph). That's why you don't always see the massive wreck you expected after sitting in traffic for so long. It's just the result of varying speeds, women drivers, and probably some asshole on a cellphone.
NOTE: Joking ladies. I have two or three female friends so you know I'm not sexist. I have nothing against women drivers. Just my wife, because she sucks at driving. She couldn't keep her car off the sidewalk if it meant a lifetime supply of Little Debbie snack cakes.
Traffic Flow Video [notcot]