Your comments on jerks and traffic jams

Yesterday’s post generated a lot of great conversation, so I thought I’d respond to some of the interesting points brought up in the comments section. First, are platoons (chains of several cars that are traveling close together) really that bad?

Anonymous said…
The thing about these “platoons” is that when you’re driving that close you’re not just watching the car in front of you but the car in front of him. You actually end up monitoring a couple of cars ahead. So if you see something happening to the car 2,3 or 4 cars ahead you start easying off and you have more time to brake when the mud really hits the fan.

Strangely enough, Dr. Appert-Rolland told me that the specific expressway she was looking at rarely had accidents, though that wasn’t the focus of her study. With such long platoons, it seems like rear-end collisions would happen pretty frequently, so maybe there’s something to the above comment. I’d be interested to know why accidents don’t happen more frequently here. When I asked her what drivers should do about this platoon effect , she mentioned not only obeying the three-second rule, but trying to look ahead if you can, as the comment above suggests. So perhaps you can fine-tune how closely you follow based on how far ahead you can see, meaning the writer of the comment below should probably stay away from bigger cars:

That’s another issue. I can’t see a damn thing from my Ford Focus when any SUV is ahead of me. I end up depending on the SUV driver’s reaction time and style, which is, generally speaking, horrid.

Obviously the three-second rule is a rough way to keep your distance larger than your reaction time. (One commenter brought up that reaction times are actually less than one second. Anyone have any evidence for that?) But sometimes even that rule needs to be broken, bringing us back to the first paper the article mentions:

the three-second rule means that most people would never be able to enter the highway. If you attempted to merge, you would break everyone else’s three second rule behind you and everyone would have to slow down to let you merge. So you can’t say “never” because your highway would be at capacity in no time at all.

This comment brings up the fact that any small disruption in the flow of traffic is going to ripple throughout the rest of the system. The closer your headway, the more the car in front of you is going to affect how you drive. If you’re, say, driving through the Mojave at midnight, you’re not going to worry about the tail lights you can see half a mile ahead of you, but if you’re tailgating a car on the 405 south, you’re going to slam on the brakes every time you see your leader’s brake lights flicker. So I’d argue that if everyone followed the three-second rule, merging traffic would have less effect on the rest of the freeway. Appert-Rolland also mentioned that it might help to display a “suggested speed” that reflects the average speed, helping drivers going too fast or slow to choreograph their driving. She pointed out that simply having shorter headways gets more cars through. The problem is that when you have a heavy flow, there’s a higher risk of a jam. And once you’re in that jam, she said, it’s really hard to get out of it. If you could somehow homogenize the flow, you’d smooth out the bumps…theoretically. According to Minnhagen’s research, homogeneity isn’t actually that helpful.

Yahktoe said…
For anybody interested in reading Petter Minnhagen’s paper (or at least an abstract of it), you can find it here:

Flow improvement caused by agents who ignore traffic rules

I think it’s worth noting how stylized the pedestrian rules are in this study. I haven’t sprung for the full paper, but I find myself wondering if maybe the rules themselves could have been changed in such a way as to make rule abiding the optimal “solution.”

A comment after my own heart: I often wonder whether the proverbial spherical cow eats grass and moos. So here’s a bit more detail on the rules of this model world. So we’ve got our pedestrian-only street, divided into a grid. Each pedestrian is randomly assigned to move either up or down the street, with each square representing a possible move. You try to move forward if you can, but if there’s someone in that square, you can move sideways. If you’re a good, rule-abiding citizen, you always try to go right first. If that square’s occupied too, then you can go left. But if you’re a jerk, you’re behavior isn’t so rigid. You toss a coin to decide whether to go left or right first. You’ve got more freedom.

Even though we’re talking about pedestrians here, and in a computer simulation at that, it’s still very surprising that the best possible situation where you have to get a bunch of people to move in a coordinated way is to have quite a few of them acting outside of the norm. Minnhagen said he was completely astounded by the results. “I was also fascinated when I was a kid by occasions when you were able to break the rules,” he told me.
Peter Minnhagen’s expertise is, broadly, in statistical mechanics, and he said that this was his group’s first big foray into traffic problems, inspired when someone brought up the question while they were tossing around ideas. (Sign me up for that group!) Cecile Appert-Rolland studied fluid dynamics for years before she delved into the physics of tailgating. So what I like about these two papers is that they apply the physics approach the infuriating and seemingly irrational phenomena in every day life. I’m definitely looking forward to seeing better models, but I’m also just fascinated by the fact that you can look at these systems with a physicist’s eye and start to get some answers.

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