I was in Los Angeles recently, visiting family. While riding in the car on our family outings, we -- shockingly -- encountered considerable traffic on some of the major surface streets, as well as on smaller parallel-running streets that many drivers apparently thought would offer a respite from the snail's pace they had been experiencing.
These slow-downs on the road reminded me of what Cornell University applied mathematician Steven Strogatz describes in his book Sync as the "emerging field of traffic physics" (p. 269).
Strogatz discusses two research teams -- one consisting of Dirk Helbing and Bernardo Huberman, and the other of Boris Kerner and Hubert Rehborn -- that have come up with interesting findings within the past decade.
Given that we think of physics as dealing with, well, physical systems, and traffic involves human behavior, how can the two be joined? As noted in this Science News article, scientists have drawn analogies in their research between traffic flows, on the one hand, and physical phenomena such as phase transitions between liquids and solids, or the clumping behavior of granular substances such as sand or salt, on the other. The field of physics under which these issues would be studied is that of statistical physics.
Other magazine-type articles on the physics of traffic, geared to the layperson, are available here and here. You might want to read one or more of these articles before you go out on the road the next time. They might not make you feel any better when you encounter traffic, but at least you'll probably have a better idea of the underlying dynamics of what's going on!