Watch this short video to find out.
It’s quite amazing how with only 20 cars driving around a circle, all supposed to be doing 30kph, a wave of backup slowly flows around the ring of cars.  Quite quickly the spacing between cars starts to change.  As some cars speed up, they create gaps.  Is the cars behind them speed up, other drivers speed up too.  Soon, a bunch of faster-moving cars catch up to slower moving cars.  Only when all the cars before the driver ahead of the slow pack move up does the driver leading the slow pack speed up.  Still confused?  Just watch the video.  Its only 30 seconds long, and shows what happens really clearly.
Still not an excuse for being late to work though. . . . .

http://www.maniacworld.com/shockwave-traffic-jam-test.html