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Why does the concertina effect happen in motorway traffic jams?

Why does the concertina effect happen in motorway traffic jams?
Mrs Balfour from Hampshire (Age 45-54)


5 Responses

  1. In my experience sitting in a traffic jam is quite a regular thing and very often there is no reason for it. The process is entirely down to humans.

    You can observe that cars travel quite close together and when one car touches the brake pedal the lights at the rear illuminate warning the driver behind that the driver in front has applied the brakes. But how much? it’s not possible to determine how hard the car in front brakes in a split second so it’s likely that the second car brakes a little harder than the one in front to avoid hitting it.

    This effect it passed along the line with the effect that the speed is reduced along the line until either the line stops or there is a suitable gap in the traffic that lessens the effect. The net result is that cars can actual stop traffic by just touching the brakes.

    Stopping this is fairly easy, leave a large enough gap to the vehicle in front or invest in a car with radar sensing speed control which will stop you getting too close.

  2. By concertina effect I assume you mean the phenomenon of stationary traffic still occurring long after the cause has been removed. The “shockwave” of congested traffic is similar to the action of trying to pour sand, rice or beans or some similar material into a conical pourer such as a funnel. Tipping in the material at a slow rate will see the material exiting at the same rate. When the rate of tipping increases, there will come a time when the capacity of the funnel cannot cope with the influx and it starts to back up in the funnel. Even when stopping tipping, the level in the funnel will then take some time to drop.
    In addition to this, when traffic is at or near capacity, a succession of drivers slightly over-reacting to the brake lights of the vehicle in front will cause the “concertina” effect. This will happen on any road where the capacity of the road is being reached, not just motorways. As traffic levels increase and the capacity of the road is reached, the traffic ceases to flow freely and becomes unstable. This usually continues until the number of vehicles decreases and orderly flow is resumed.

  3. It is the reaction time of each motorist that creates the lag. You can measure this in traffic queues at traffic signals and seems tobe about 1 second per vehicle.

    If you are in a queue at lights that are red and you can see the lights change to green, count slowly until you are able to move forward and the number will probably be the same as the number of cars that were in front of you in the queue.

  4. Leaving a gap also causes others, (who think they are getting far by getting one car further in another lane) to cause a hold up in this other lane.. they cause the car they jump in front of to brake causing this above effect.


    Create more lanes and better infustructure for the people generating the money for these roads in the first place and stop treating us like ants in an ant farm or cattle on a farm.

  5. Traffic jams are almost exclusively caused by humans failing to properly anticipate when to go and failing to go at the appropriate speed, especially at traffic lights or lane merging points. For example, when waiting in a column of vehicles at the traffic light, people tend to start moving according to the car ahead instead of the actual traffic light. In theory (and practice, when the drivers understand the situation properly), all cars should start moving SIMULTANEOUSLY, increasing the real throughput UP TO SEVERAL TIMES, even without excessive/dangerous acceleration or breaking the speed limit. For example, it is possible to get 50 or even more vehicles through a traffic light, where just 15 vehicles regularly pass during one green light period. Some women tend to be slower than men in similar situations, leading to hate outbursts by less tolerant/patient men or even other women. But many women drive just like men and most men don’t drive nearly as efficiently as is actually possible in dense traffic situations. Automatic driving systems would basically make traffic jams history with the same number of cars on the roads (while improving safety, fuel consumption, exhaust emissions, noise, brake wear, etc.). Also, driving a 17′ long car alone contributes to throughput decreases (even with optimized driving).

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