Joel Shurkin in Physics Buzz (Image credit: Alexandre Dulaunoy via flick):
The problem of random traffic jams on most roads, said Berthold Horn, an electrical engineer and computer scientist at MIT, is sometimes described as an issue of fluid dynamics. Other scientists point to chaos theory and fractals to explain the phenomenon.
An analogy Horn uses is dilatant fluid, a fluid that gets thicker as stress is applied. For example, if you put enough corn starch into a swimming pool, you could walk across it on the surface. The content of the pool would remain a liquid but it would thicken under the pressure of your step. YouTube is full of examples, Horn said.
The water in the pool is called a Newtonian liquid because no matter what you do with it — shake or stir — it remains a liquid. Once you add the corn starch, it becomes a non-Newtonian liquid and solidifies under pressure.
Ketchup, incidentally, is the reverse. It is mostly a solid in the bottle until you shake or squeeze it, then it becomes a liquid. The non-Newtonian properties of ketchup are the reason it is so hard to get it out of a glass bottle.
Now, think of the stream of traffic as a liquid. When you reach a certain number of cars in a certain area simultaneously, Horn said, the traffic “thickens,” and everyone slows down because everyone is reacting to the car in front of them.
Horn, thinks he has found the solution in luxury cars–adaptive cruise control, which uses radar to monitor the car ahead of you. If that car slows down, so does yours, in direct proportion to what the other car is doing.
What Horn wants to do is watch the rear as well.
He calls it bilateral control: To keep traffic moving, you also have to look behind you.