Why does traffic sometimes jam up on a road for no reason whatsoever? Japanese scientists from the Mathematical Society of Traffic Flow (really) asked a bunch of people to drive around a circle at constant speed. Watch as jams materialize out of nowhere and then "ripple" back through the circle like a shockwave:

         

Here's the article. One theory: "I suspect the trigger would either be a particular driver who was more nervous than the rest, or a particular location on the circle where the capacity was slightly lower." The first makes sense—a quick tap on the brakes by one motorist can reverberate down the line until everything grinds to a halt. I wonder if it'd be possible to create a computer-guided "conveyor belt" for cars in certain high-jamming areas—on LA freeways during rush hour, say—to maintain a constant distance between cars. Think of the productivity gains! Though presumably most drivers would be loath to give up the feeling of control, even if you don't actually have much control being wedged in bumper-to-bumper traffic.