Let's talk about traffic.
Professor Yuki Sugiyama [who runs the Mathematical Society of Traffic Flow at Japan's Nagoya University] demonstrated that traffic jams can be a sort of wave — known as a "stop-and-go-wave." Like stadium waves, these are not like the usual sort of waves that physicists study. But they do appear like a sort of density wave within the flow of traffic as cars join the jam at the back and others pull away at the front. Sugiyama found that the critical number of cars on the road [for these waves to form] is 40 per mile. When the weight of traffic is more than this, the flow becomes "unstable" and the natural fluctuation in our driving style is enough to cause these waves to start up. No obstruction is needed — it is just a matter of time before a traffic wave will form.