The Physics of Traffic Gridlock

Brian Schwartz is vice president for Research and Sponsored Programs at The Graduate Center, co-director of the New Media Lab, and a professor of physics at Brooklyn College. Recently, he has been researching traffic gridlock in urban environments with physics doctoral student Lei Zhou by simulating traffic patterns on computers in the New Media Lab, which involves using a "Cellular Automaton" model and 3D-imaging software. Following the laws of physics, Schwartz is able to determine what factors make traffic flow more smoothly or, alternatively, create gridlock, and he can make predictions accordingly. Folio talked to Schwartz about this research that may offer solutions to every commuter's daily nightmare.

We all understand that there are molecules and atoms, and when they get close together, they interact with one another. And then they can do very strange things. The molecules can end up as a gas, they can become a liquid, or they can become a solid. Cars are like atoms: when they get close together, they begin interacting, and they interact in a very similar way. There's a law of physics that is almost a one-to-one analogy with the law of cars. In physics the law says that no two electrons can be in the same place at the same time. Similarly, no two cars can be in the same place at the same time. And so, you can begin setting up rules about how cars behave, just like you set up rules for molecules or atoms. You can then begin applying those rules to real situations of traffic flow.

There are three kinds of traffic flow on a highway. The first is when your car has a lot of freedom. You can go as fast as you want and change lanes whenever you want. This is free flow, and it is very much like the behavior of the molecules of a gas. Next, you can be in moderate traffic that is moving well. It's now more difficult to change lanes, and there's a car in front of you and a car behind you. You can't suddenly speed up to 70 miles an hour. But the traffic is flowing beautifully. In that case, it's very much like water. The third situation is a traffic jam. And there, the cars are not moving, and it's very much like ice. In physics, we call these phase transitions. You can go from steam to water to ice, and it's all made up of the same molecules, yet it's three different states of the same matter. And it's the same with traffic.

We're able to devise rules using the Cellular Automaton Model. The model is really very simple. You take a mile of road and divide it into so many 'cells,' or 30-foot units. Then you can answer the question, 'Is there a car in this cell or not?' with a yes or no. A car can move one cell at a time, or two, or three, depending on the speed. If there are no cars in any cell in front of the car, I can let it go to any speed it wants. But what if there are other cars? Then rules come into play. If the cell in front of it is occupied, then it can't go there until the other car moves. There can't be two in one cell, very much like atoms. So, you can let the system run and really predict and show how traffic jams occur. What really matters in the system is the density of cars: how many cells are filled, and how many are empty.

Of the many topics that you can research in physics, people love transportation. It's something that they all share, like the weather. Everyone has stories about potholes and excruciating commutes. Also, my younger brother is "Gridlock Sam" (the former traffic commissioner under Mayor Koch and Daily News columnist, who is credited with inventing the word "gridlock"). My brother and I will often do radio programs on traffic. We're a team, like the guys on "Car Talk." What's interesting about traffic, unlike weather, is that if you predict the traffic, you can influence the traffic.

What Lei Zhou and I are doing is looking at urban traffic, where there's a grid, and making rules that will make it flow better. On the basis of the calculations, we're making suggestions to the City for how to control the lights and how to control traffic behavior. As it is now, the City uses good common sense, but a lot of things that you discover are not common sense. They are contrary to intuition. From our research, we know that the traffic gets unstable when there are too many cells filled up and there are too many cars in a certain length of road. And if the system is unstable, it can flip from one phase to another, like the changing of water into ice. So you want to keep the system away from these unstable points where it can very quickly switch. We tell them to keep the density below a certain level or there could be a phase change.