# Traffic Jams are Caused by ... Too Many Cars on the Road!

This is a familiar experience for most of us: getting stuck in a traffic jam only to find out later that there is no obvious cause (like accident, construction work or bottleneck).

Well, Japanese scientist Yuki Sugiyama of Nagoya University has finally found the answer: the traffic jam is caused by ... too many cars!

In the New Journal of Physics a study by his group explains why we're occasionally caught in jams for no obvious reason.

The real origin of the snarl up often has nothing to do with obvious obstructions such as accidents or construction work but is simply the result of there being too many cars.

The team discovered the importance of traffic density by applying techniques to model the movements of lots of particles to real-life moving traffic. The research shows that even tiny fluctuations in car-road density cause a chain reaction which can lead to a jam.

The team also studied cars driving around a circular track with a circumference of 230m. They put 22 cars on the road and asked the drivers to go steadily at 30km/h (19mph) around the track. While the flow was initially free, the effect of a driver altering his speed reverberated around the track and led to brief standstills.

Prof Sugiyama says, "Although the emerging jam in our experiment is small, its behaviour is not different from large ones on highways. When a large number of vehicles, beyond the road capacity, are successively injected into the road, the density exceeds the critical value and the free flow state becomes unstable."

Update 3/22/08 - Here's a video clip that illustrates what happened:

As someone who commutes 150 kms every day, I believe this research - however obvious it seems - is useful. When were drive on straight areas, it doesn't matter if there is 10 cars or 100 cars: the flow of traffic remains constant. If you throw in a distraction like police lights on the opposite side, or a change in the road like a sudden curve (even a slight one), the affect of a single person braking radically alters the speed of each successive car. The flow moves like boa, stretching until it hits a bend and than contracting until it makes it around the bend. One panicky, brake-happy driver on an unfamiliar road can drop my speed from 100 km to 70 km in seconds - and thus, every car behind and beside me; add to that the lane changers, who double the volume of fast lanes in seconds, and you have massive slow-downs with or without more cars.
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Almost this exact same thing was published something like 20 years ago in Scientific American (couldn't find link to article).
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ObsidianPunk is right. Give them some credit--99% of what they do and know isn't apparent to you or me. What we do see can seem obvious or simple because someone along the line simplified it for general consumption.

And just because it all seems simple doesn't mean it is. This is true in any job that requires any knowledge or skill. Carpenters may hammer a nail and make it look easy. A novice may think it's easy, but will either bend the nail, smash their thumb or put the nail in the wrong place. End of detour.

Notice he talks about particle effects. Not being a scientist, I'd guess this means it's not about individual drivers, but a mass of cars behaving in a particular way, regardless of drivers. What can we do with this information? The same thing scientists do with hydrodynamics I suppose. Learn how particles/molecules behave moving through a pipe and you figure out how to make it go smoother.

MoonCake, the article wasn't talking about accidents clogging up traffic. It was about jams for no apparent reason.
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