Shockwave traffic jam

Traffic that grinds to a halt and then restarts for no apparent reason is one of the biggest causes of frustration for drivers. Now a team of Japanese researchers has recreated the phenomenon on a test-track for the first time.

The mathematical theory behind these so-called “shockwave” jams was developed more than 15 years ago using models that show jams appear from nowhere on roads carrying their maximum capacity of free-flowing traffic – typically triggered by a single driver slowing down.

After that first vehicle brakes, the driver behind must also slow, and a shockwave jam of bunching cars appears, travelling backwards through the traffic.

The theory has frequently been modelled in computer simulations, and seems to fit with observations of real traffic, but has never been recreated experimentally until now.

Shockwave traffic jam recreated for first time

3 thoughts on “Shockwave traffic jam

  1. ross

    That’s pretty cool.
    If you want to run this experiment for yourself but are on a budget, I recommend using a yogurt-container lid and a couple dozen well-trained ants.

  2. Scott K

    I wish they would have added a second trial in which the drivers could speed up or slow down to try and maintain the correct spacing. As it was done, I think the drivers were told to try and maintain a constant 30 kph. I wonder if the traffic would flow better longer or would traffic jam problems develop sooner?

  3. ross

    That is an interesting question… With the cars starting from a standstill and in a line (not initially spanning the entire track): The front driver wouldn’t have a limit on his speed until he wraps around and is just behind the “last” car… within reason, the front driver is likely to assume that the car directly behind him will maintain their proper distance (by the 2nd car speeding up). Likewise, the 3rd car would speed up to maintain proper distance with the 2nd car… and so on (I doubt drivers would slow down to maintain proper distance with the car behind him/her) . If all the cars are able to get up to the speed that the lead car set while maintaining comfortable distance between eachother, then I think they would be fine. However, if the lead car goes too fast, then he will wrap around and be slowed by the “last” car. The 2nd car would then slow down, followed by the 3rd car, and so on. I think pretty quickly from this point the cars would converge to a near-ideal speed: one that maximizes the average speed of the cars as a group, while letting drivers maintain comfortable distances between eachother.
    So I think the traffic would flow fine (possibly after a short period) because of the additional control the drivers are given to determine their own speeds.
    That would be an interesting experiment to see.
    It’s late and I’m tired, so hopefully that wasn’t absolutely complete rubbish 😀

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