Traffic Flow

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For the last month or so I’ve been wondering about traffic flow, or specifically, the question of how to maximize its efficiency.  I’ve been trying to decide between two models, which I will dub the American model and the Chinese model. In the American model there are a bunch of rules that are broadly obeyed, such as staying within one’s lane, leaving one car length of space between you and the nearest car, and obeying traffic lights. In the Chinese model, these rules are more like guidelines, so traffic flow is much messier. You might have your own opinion (if so, I’d be happy to hear it!), but I thought about this for a while and it’s not intuitively obvious to me which model leads to more efficient traffic flow.

Fortunately for me, a team of physicists was also wondering about this same problem, and they constructed a model with cellular automata to settle the question. Their paper is available here. It turns out the Chinese system is more efficient than the American one, but the most efficient  “occurs when a small fraction of [rule-] ignorers are present within a majority of [rule-] abiders”. So that settles that question.

They only explored the question of efficiency, however. My intuition, as well as Professor Appert-Rolland at Paris-Sud University, suggests that breaking the rule about leaving one car-length of space will cause lots of accidents. So I looked up the statistics, and China does indeed seem to be a more dangerous place to drive. In 2008, there were 73384 traffic fatalities in China, corresponding to 4.3 fatalities per ten thousand motor vehicles on the roads. In 2008, there were 37261 traffic fatalities in America, corresponding to approximately 1.6 fatalities per ten thousand registered motor vehicles (this one’s using 2007 data, but the number is fairly constant year-to-year). It’s not quite so simple, though, since American cars are on average much larger than Chinese cars, and size should make the fatality rate decrease. And there aren’t reliable statistics on non-fatal accidents in China, so I can’t do that comparison. Let’s adopt the tentative conclusion that relaxing the rules leads to more efficient traffic flow, but the jury is still out on safety.

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