How traffic jams are made

via Boing Boing

A couple of years ago, Cory posted a really interesting story about the mathematics behind seemingly cause-less traffic jams. It’s pretty interesting. Shorter version: The researchers think jams like this are caused by one person braking, and the response to that slow down moves through dense traffic in a way that is mathematically very similar to the shock wave from an explosion. Once you have enough density of cars on a road, jams are inevitable.


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  • jonno1

    Hmm, no surprises here. I was taught this at engineering school about 40 years ago (basic wave theory), and it’s visible on the Auckland motorway every day. I notice the narrator says “for the first time… real life experiment” giving the impression that it’s some great new breakthrough in understanding. But I seriously doubt that it’s the first time anyone has carried out such an experiment, and it will certainly have been modelled many times. Heck, I may even have done in Fortran IV. Disclaimer: I’m not a traffic engineer so don’t have specific knowledge in this field.

  • Anonymous

    This is why passing lanes are closed off during holiday peak traffic – motorists braking when merging slows the whole lot down.

    • jonno1

      Agreed peterwn. It seems counter-intuitive but it works. Of course, a further problem is lane reduction at random intervals…

  • Gravedodger

    So far so good thats sorted, now why does a single vehicle I have followed for 10 Kms at 85 KPH suddenly accelerate to 110 in a passing lane then slow back to 85 when the lanes merge again.

    • Paul Rain

      Because you aren’t allowed to shoot to kill in such a situation.

  • Anonymous

    See it almost everyday on the southern motorway at Takanini. Someone hits the picks going over the rail over-bridge and it eventually clogs back to Manukau.
    I remember seeing the same scenario using a computer model some years ago.

  • jay cee

    see it a lot when a car or cars in an accident or broken down  being cleared to the side of the road. nothing to see as it has all been cleared away but because traffic was initially held up at that point drivers still slow to see what happened.bloody rubberneckers !!