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Traffic Fluid Dynamics


I drove from Oregon to Washington a few days ago, at the end of the Fourth of July weekend. Naturally, traffic was worse than normal. The jams came in waves, and lasted 20-30 minutes each. These sorts of jams are, practically speaking, guaranteed to create themselves in dense enough traffic and for no good reason. Whenever a car breaks, the cars behind break, sending a shock wave of stopped cars through the highway. If you’ve ever seen traffic jams that stop your car for ten seconds, then move forward for ten seconds, and then stop again, you know what I mean.

An electrical engineer named William Beaty knows what I mean, too. He’s dedicated a website to the phenomenon, and on it, he explains how to prevent the process:

Once upon a time, years ago, I was driving through a number of stop/go traffic waves on I-520 at rush hour in Seattle. I decided to try something. On a day when I immediately started hitting the usual “waves” of stopped cars, I decided to drive smoothly. Rather than repeatedly rushing ahead with everyone else, only to come to a halt, I decided to try to move at the average speed of the traffic. I let a huge gap open up ahead of me, and timed things so I was arriving at the next “stop-wave” just as the last red brakelights were turning off ahead of me. It certainly felt weird to have that huge empty space ahead of me, but I knew I was driving no slower than anyone else. Sometimes I hit it just right and never had to touch the brakes at all. Other times I was too fast or slow. There were many “waves” that evening, and this gave me many opportunities to improve my skill as I drove along.

I kept this up for maybe half an hour while approaching the city. Finally I happened to glance at my rearview mirror. There was an interesting sight.

It was dusk, the headlights were on, and I was going down a long hill to the bridges. I had a view of miles of highway behind me. In the neighboring lane I could see maybe five of the traffic stop-waves. But in the lane behind me, for miles, TOTALLY UNIFORM DISTRIBUTION. I hadn’t realized it in the past, but by driving at the average speed of the traffic around me, my car had been “eating” the traffic waves. Everyone ahead of me was caught in the stop/go cycle, while everyone behind me was forced to go at a nice smooth 35MPH or so. My single tiny car had erased miles and miles of stop-and-go traffic.

~William Beaty

I tried out the process during my drive. It’s tougher than it sounds. The main problem is that the flow of the traffic picks up and slows down even when the cars are moving, not just when they’ve stopped. This makes it difficult to measure the average speed of the traffic, and so I found myself going too fast or too slow. Secondly, cars in the lanes next to me would constantly slip into the large gap that I left before me, forcing me to stop again. Cars behind me decided I was moving to slowly, and pulled around me solely in order to be the first ones to enjoy stopping at the next jam.

This joke really ... tanked. Heh. Get it?

A secondary cause of traffic jams.

It’s funny that Beaty also lives in the Seattle area, since that’s where the jams were worst. The drivers in that area are not known for their skill and understanding. Or, as John Vance, a commenter on the Boing Boing post that brought this traffic pattern to my attention, said, “It’s another one of those beautiful practices that relies on human kindness and rationale to succeed on any appreciable scale–so it is doomed to failure.”


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