This chart is basically a re-drawing of the one that I put up last week, depicting flow rate vs. density for a traffic jam. The difference here is that it depicts a non-linear time sequence, showing how traffic flow increases, destabilizes, and then falls into a separate “congested” regime with lower flow rates.

Coincidentally, it is precisely this kind of behavior that ramp meters seek to control. If ramp meters can keep flow rates from plummeting into the “post-critical” phase of the cycle, then everyone traveling on the freeway is better off. In other words, when it comes to freeway behavior, congestion is a one-way street (topologically speaking, of course).

PS Image comes from Philip Ball’s interesting book, Critical Mass.