The only thing more frustrating than creeping your way toward the site of a bottleneck on the highway only to discover the accident is on the other side of the median are the times when you make it through and discover, as far as you can tell, nothing was holding up the traffic. Japanese researchers have now demonstrated that the “nothing” may in fact be the traffic crossing a threshold of density of cars on the road. Too many cars means that small slow downs by a few drivers equals up to big backups miles away.

This idea has been around in physics for some time under the model of multi-particle interactions. What the researchers at the Mathematical Society of Traffic Flow did this time that was different was to put together a controlled experiment. Drivers were placed around a circular track and told to keep a uniform spacing and a slow, consistent speed. Everything starts well enough, but over time, because the drivers are human, their speeds fluctuate slightly and the bunch-ups quickly appear.

It sounds like a lot of common sense, but is valuable research because it better informs how to construct theoretical models for future studies. How the problem can be managed is still anyone’s guess, but as long as our understanding grows, we’ll only get closer.