Although freeway traffic flow is very complex, science is beginning to reveal its dynamics. It turns out to have distinct states, like the gas, liquid, and solid states of matter. When traffic moves like a gas, the cars are far enough apart that each one can move freely without much affecting its neighbors.
Drivers instinctively maintain large separations, and traffic flows at maximum speed.
When things get a little busier, cars slot together into clusters that travel only a little slower than they do in the gas state. The clusters are like condensed droplets of liquid, and they are separated by intervals of gaseous traffic. If traffic gets even heavier, another transition happens. The traffic enters a thick, viscous state like honey or tar. If it gets much heavier you have gridlock, the solid state of traffic flow.