Motorway traffic jams?

Question

Why is it that you can be crawling along at 10 MPH for hours and then suddenly as if by magic it clears?

Answer 1

It's the knock on effect of people changing lanes inconsiderately, and people braking when they don't need to brake (or braking too harshly). All of this results in people behind them braking even harder, which results in people behind them braking harder still till you get this domino effect which, after 5 or so cars will result in a tailback.

People also take time between seeing the car in front moving off and them moving off themselves. So again, the domino effect would be that the first car moves off, the second car takes a bit of time to move off, and the third takes a bit more than that, and so on, till by the time the 5th car down the line moves off it's held off more traffic behind it.

So it could all start with a busy junction where people would brake to turn off, or change lanes suddenly to take the exit that could start the chain reaction. All it needs is something for someone to brake harder than he needs to (or brake even when he doesn't need to - you do slow down if you just take your foot off the accelerator) and the domino effect is started and you get a tailback.

Which is why I really do not agree with this proposal to open up hard shoulders to "ease congestion". What would happen is that for the few miles that you have continuous hard shoulder, traffic would run smoothly, then when the hard shoulder stops, or there's a turnoff and people on the hard shoulder would have to change lanes, a bottle neck situation ensues and this triggers the chain reaction that will lead to a tailback. The tail back might even extend back into the few miles where the hard shoulder helped smooth the traffic and clog that up too. Pointless if you ask me.

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Thanks youngperksy56; I was looking for that word and just couldn't get my finger on it...

Answer 2

This is a phnomenon which physicists sometimes call drift theory. Basically, it's because at some time before you reach the"bunch" there was an incident. This may have been anything from a major pile up to just some dingbat doing a bad lane change.
The point is that the traffic had to stop, or come close to stopping. If it's a busy road then a lot of vehicles will stop quickly. When the obstruction is cleared, or the idiot has diven off, the traffic will start up again.
Because motorway traffc is not all coupled up like a train the cars all start at different times. Hence, you get a crawling slow re-start from the original stoppage.

Answer 3

You can get phantom traffic jams.

If people are driving too close to the car in front, they are likely to have to break (sometimes quite hard) to avoid hitting them. If the person behind them is too close, they will then have to break even harder, and so it goes on until farther down the line there is a jam.

There are other reasons of course. A minor accident which stops the traffic, and then is cleared when the cars move to the hard shoulder. A major accident where the motorway is closed completely....

Answer 4

because bell ends like to slow down and look at nice flashy blue lights on the other side of the road /carridgeway instead of just getting on with the job.

Answer 5

It's the Governments failure to provide adequate roads as they take six sevenths of the tax motorists pay as fun money!
RoyS

Answer 6

k2 has explained perfectly except for the name, its actually the concertina effect, something that americans dont suffer anywhere near as much as we do, because almost everyone drives auto gearboxes