Chart of the Day: Flow Rate v. Density (on Freeways)



This chart comes from Philip Ball’s interesting book Critical Mass, which describes complex non-linear dynamics using real world examples. There’s one fascinating chapter on traffic jams, and I’m going to post a few of the charts from it.

For example, the overall volume of a freeway is a combination of speed + density, something that is here called “flow rate.” (I.e. 100 cars per hour can equal 100 tightly packed cars traveling at 30 mph or 100 loosely spaced cars traveling at 60 mph.)

As density increases, the flow rate increases as well until you reach a critical point, whereby the slightest perturbation in the system will cause a dramatic decrease in speeds to cascade through the system, average speeds to plummet, and the overall flow rate to decrease into a second kind of “basin of attraction.”  This is a classic example of non-linear dynamics, and is one of the things that makes traffic jams so predictably unpredictable and therefore fascinating.

(Stay tuned for more next week…)



Bill Lindeke

About Bill Lindeke

Pronouns: he/him

Bill Lindeke has writing blogging about sidewalks and cities since 2005, ever since he read Jane Jacobs. He is a lecturer in Urban Studies at the University of Minnesota Geography Department, the Cityscape columnist at Minnpost, and has written multiple books on local urban history. He was born in Minneapolis, but has spent most of his time in St Paul. Check out Twitter @BillLindeke or on Facebook.