It turns out that traffic is a deceptively complicated problem. It could be said to resemble molecular physics, in fact, since it’s a system of individual particles interacting in complex ways. Except, with traffic, the particles have minds of their own.

There are two kinds of traffic flow. In uncongested, stable flows, cars can move at or near the speed limit, and individuals are able to move in and out of lanes or enter the highway smoothly. Then, there’s what traffic experts call the “unstable regime,” what laypeople refer to as stop-and-go traffic. What scientists have figured out over the past decade or so is when and why traffic shifts between the two.

“We see in our models that traffic becomes unstable when the number of cars [passing a specific spot] per lane per hour reaches between 2,000 and 2,500. At that nominal capacity level, traffic is very likely to become unstable,” says Hani Mahmassani, a traffic scientist at Northwestern University in Chicago.

It’s entirely possible for 2,000 cars traveling at ordinary highway speeds to cross a particular line in a single lane in an hour, as long as they’re all moving at the same speed and no one tries to move in or out. But that rarely happens. Far more often, after just a few minutes at that high flow rate, something upsets the process.

Consider a classic case. A slow-moving car shifts into the left lane to pass an even slower-moving car. The car immediately behind the lane-changer has to decelerate dramatically — not just to the speed of the car in front of him, but slow enough to create a safe driving distance between them. The next car back has to slow down even more, again to give itself a cushion. This slowdown ripples back through the lane and eventually spreads into the other lanes as nearby drivers notice the sea of brake lights and reflexively slow down. Traffic researchers refer to this as a shock wave, and it can travel back for miles.

Eventually, the slower car moves back into the right lane, and the traffic speed picks up. From the perspective of a car a mile behind the instigator, there was never any reason for the slowdown.

Unfortunately, while we’ve gotten really good at understanding why traffic jams happen, our tools to prevent them are pretty limited. Some experts are proponents of ramp metering. That’s when each on-ramp contains a traffic light to control the number of cars entering the highway.

“Two thousand cars per lane per hour is great, but it can’t be maintained. And when the breakdown occurs, flow rate immediately drops to around 1,200,” says Mahmassani. “The ramp meters are intended to keep the rate around 1,750 or 1,800, which is more stable.”