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Pit problem
THE LAST WORD Concertina cars I was driving down the motorway the other day when the traffic suddenly came to a standstill. After 10 slow minutes of being mostly stationary, the traffic started moving again and was back up to full speed almost immediately. There was no visible reason for the standstill, no accident or junction in sight, and it was slightly later than morning rush hour. Can anyone tell me why traffic bunches up like this for no apparent reason?
n In open, free-moving traffic, each car is basically autonomous and can travel as fast as its driver wants to go. In denser traffic, there is interaction between vehicles, and if one car slows down then the one behind must also slow down. When the traffic reaches a certain critical density, a “shock wave” can spontaneously travel back through it. This is because when one driver brakes gently, the driver behind will choose to slow down more markedly. The effect becomes more pronounced as it works its way backwards. Meanwhile new traffic keeps on arriving at the same rate. With nowhere for it to go, it comes to a grinding halt. The front end of the jam gradually clears, and when cars at the back finally get moving again, the road in front of them is virtually empty, and drivers wonder what the problem was. Traffic congestion is hard to model because it is very nonlinear and dependent on human
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The merged wave brought more reaction times. If drivers had a traffic to a halt as it moved up the reaction time of zero and could slope past me and disappeared respond instantly to changes over the crest of the hill. in the flow, an entire highway Once traffic had cleared the could start and stop together, like point where the lorry switched soldiers on parade. But we know from watching cars take off at traffic lights that there is roughly “We know from watching a 1-second delay between each car cars take off at traffic starting to move. It is probably no lights there is a 1-second coincidence that the critical traffic delay in cars starting” flow that results in a complete lanes, it began to speed up and standstill is about one vehicle unbunched, creating a third per second. “acceleration” wave. This, too, Hugh Hunt travelled up the hill, enabling Trinity College stopped cars to move and quickly University of Cambridge, UK reach the speed they were moving n While travelling towards at before the bunching occurred. southern England on the M5 This third wave also disappeared motorway, I saw how a bunch-up over the crest behind me. in the traffic evolved, from start For anybody who had not seen to finish. This was fascinating to the lorry’s manoeuvre, or for me as a scientist. anybody over the crest behind Where it occurred, the me, there would have been no motorway sloped down for about apparent reason for the stoppage. a mile, then levelled off. Just as Richard Nolan I started down the slope, a lorry Cratloe, Clare, Ireland about a mile ahead indicated it n When a smoothly moving wanted to move from the leftmotorway is nearly at full hand lane into the centre lane. This caused traffic behind to start capacity, glitches resulting from a driver changing lanes or braking bunching, presumably because will cause cars behind to brake, drivers eased off the accelerator creating a mass of slow-moving or (I saw no brake lights). This stopped cars. If the number of cars bunching travelled backwards that joins the back of this mass up the slope, like a wave. matches or exceeds the number of When the lorry changed lanes, cars leaving over the same period, a second wave started moving up the mass will persist and even the slope, presumably when grow. It may do so for extended drivers braked (their brake lights periods, dissipating only when came on). This second wave traffic flow decreases for long travelled faster than the first, merging with it eventually. Where enough that more cars leave than join, slowly dissipating the mass. the waves merged, traffic halted.
New Scientist retains total editorial control over the content of The Last Word. Send questions and answers to The Last Word, New Scientist, Lacon House, 84 Theobald’s Road, London WC1X 8NS, UK, by email to [email protected] or visit www.last-word.com (please include a postal address in order to receive payment for answers). For a list of all unanswered questions send an SAE to LWQlist at the above address.
A similar phenomenon occurs in data networks such as the internet: during periods of congestion, queues of data packets build up quickly, persist, and are slow to dissipate. I described the precise mechanism for this phenomenon in my PhD dissertation. Srinivasan Keshav School of Computer Science University of Waterloo Ontario, Canada n To kill these waves in dense highway traffic, leave six to eight car lengths to the next vehicle, and as soon as you can see the brake lights of cars further along go on, take your foot off the accelerator and coast. If the car ahead of you regains speed, you can easily catch up, so avoiding a general slowdown. The cars behind will have slowed down only slightly while you coasted, so they can also speed up when you do. The idea sounds good on paper, but unfortunately other drivers may not be able to comply as the usual habit is to keep too close to the car ahead. Don L. Jewett University of California, San Francisco, US
This week’s question Pit problem
My doctor says that skin tags – small growths on the skin’s surface – commonly form in the armpit. Why do they appear? Ann Garmin Glasgow, UK
Will we ever speak dolphin? The new book out now: packed full of wit, knowledge and extraordinary discovery Available from booksellers and at newscientist.com/dolphin
n In open, free-moving traffic, each car is basically autonomous and can travel as fast as its driver wants to go. In denser traffic, there is interaction between vehicles, and if one car slows down then the one behind must also slow down. When the traffic reaches a certain critical density, a “shock wave” can spontaneously travel back through it. This is because when one driver brakes gently, the driver behind will choose to slow down more markedly. The effect becomes more pronounced as it works its way backwards. Meanwhile new traffic keeps on arriving at the same rate. With nowhere for it to go, it comes to a grinding halt. The front end of the jam gradually clears, and when cars at the back finally get moving again, the road in front of them is virtually empty, and drivers wonder what the problem was. Traffic congestion is hard to model because it is very nonlinear and dependent on human
Questions and answers should be concise. We reserve the right to edit items for clarity and style. Include a daytime telephone number and email address if you have one. Restrict questions to scientific enquiries about everyday phenomena. The writers of published answers will receive a cheque for £25 (or US$ equivalent). Reed Business Information Ltd reserves all rights to reuse question and answer material submitted by readers in any medium or format.
Last words past and present, plus questions, at last-word.com
The merged wave brought more reaction times. If drivers had a traffic to a halt as it moved up the reaction time of zero and could slope past me and disappeared respond instantly to changes over the crest of the hill. in the flow, an entire highway Once traffic had cleared the could start and stop together, like point where the lorry switched soldiers on parade. But we know from watching cars take off at traffic lights that there is roughly “We know from watching a 1-second delay between each car cars take off at traffic starting to move. It is probably no lights there is a 1-second coincidence that the critical traffic delay in cars starting” flow that results in a complete lanes, it began to speed up and standstill is about one vehicle unbunched, creating a third per second. “acceleration” wave. This, too, Hugh Hunt travelled up the hill, enabling Trinity College stopped cars to move and quickly University of Cambridge, UK reach the speed they were moving n While travelling towards at before the bunching occurred. southern England on the M5 This third wave also disappeared motorway, I saw how a bunch-up over the crest behind me. in the traffic evolved, from start For anybody who had not seen to finish. This was fascinating to the lorry’s manoeuvre, or for me as a scientist. anybody over the crest behind Where it occurred, the me, there would have been no motorway sloped down for about apparent reason for the stoppage. a mile, then levelled off. Just as Richard Nolan I started down the slope, a lorry Cratloe, Clare, Ireland about a mile ahead indicated it n When a smoothly moving wanted to move from the leftmotorway is nearly at full hand lane into the centre lane. This caused traffic behind to start capacity, glitches resulting from a driver changing lanes or braking bunching, presumably because will cause cars behind to brake, drivers eased off the accelerator creating a mass of slow-moving or (I saw no brake lights). This stopped cars. If the number of cars bunching travelled backwards that joins the back of this mass up the slope, like a wave. matches or exceeds the number of When the lorry changed lanes, cars leaving over the same period, a second wave started moving up the mass will persist and even the slope, presumably when grow. It may do so for extended drivers braked (their brake lights periods, dissipating only when came on). This second wave traffic flow decreases for long travelled faster than the first, merging with it eventually. Where enough that more cars leave than join, slowly dissipating the mass. the waves merged, traffic halted.
New Scientist retains total editorial control over the content of The Last Word. Send questions and answers to The Last Word, New Scientist, Lacon House, 84 Theobald’s Road, London WC1X 8NS, UK, by email to [email protected] or visit www.last-word.com (please include a postal address in order to receive payment for answers). For a list of all unanswered questions send an SAE to LWQlist at the above address.
A similar phenomenon occurs in data networks such as the internet: during periods of congestion, queues of data packets build up quickly, persist, and are slow to dissipate. I described the precise mechanism for this phenomenon in my PhD dissertation. Srinivasan Keshav School of Computer Science University of Waterloo Ontario, Canada n To kill these waves in dense highway traffic, leave six to eight car lengths to the next vehicle, and as soon as you can see the brake lights of cars further along go on, take your foot off the accelerator and coast. If the car ahead of you regains speed, you can easily catch up, so avoiding a general slowdown. The cars behind will have slowed down only slightly while you coasted, so they can also speed up when you do. The idea sounds good on paper, but unfortunately other drivers may not be able to comply as the usual habit is to keep too close to the car ahead. Don L. Jewett University of California, San Francisco, US
This week’s question Pit problem
My doctor says that skin tags – small growths on the skin’s surface – commonly form in the armpit. Why do they appear? Ann Garmin Glasgow, UK
Will we ever speak dolphin? The new book out now: packed full of wit, knowledge and extraordinary discovery Available from booksellers and at newscientist.com/dolphin