Pit problem
1
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. 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. 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 non- linear and dependent on human reaction times. If drivers had a reaction time of zero and could respond instantly to changes in the flow, an entire highway could start and stop together, like soldiers on parade. But we know from watching cars take off at traffic lights that there is roughly a 1-second delay between each car starting to move. It is probably no coincidence that the critical traffic flow that results in a complete standstill is about one vehicle per second. Hugh Hunt Trinity College University of Cambridge, UK n While travelling towards southern England on the M5 motorway, I saw how a bunch-up in the traffic evolved, from start to finish. This was fascinating to me as a scientist. Where it occurred, the motorway sloped down for about a mile, then levelled off. Just as I started down the slope, a lorry about a mile ahead indicated it wanted to move from the left- hand lane into the centre lane. This caused traffic behind to start bunching, presumably because drivers eased off the accelerator (I saw no brake lights). This bunching travelled backwards up the slope, like a wave. When the lorry changed lanes, a second wave started moving up the slope, presumably when drivers braked (their brake lights came on). This second wave travelled faster than the first, merging with it eventually. Where the waves merged, traffic halted. The merged wave brought more traffic to a halt as it moved up the slope past me and disappeared over the crest of the hill. Once traffic had cleared the point where the lorry switched lanes, it began to speed up and unbunched, creating a third “acceleration” wave. This, too, travelled up the hill, enabling stopped cars to move and quickly reach the speed they were moving at before the bunching occurred. This third wave also disappeared over the crest behind me. For anybody who had not seen the lorry’s manoeuvre, or for anybody over the crest behind me, there would have been no apparent reason for the stoppage. Richard Nolan Cratloe, Clare, Ireland n When a smoothly moving motorway is nearly at full capacity, glitches resulting from a driver changing lanes or braking will cause cars behind to brake, creating a mass of slow-moving or stopped cars. If the number of cars that joins the back of this mass matches or exceeds the number of cars leaving over the same period, the mass will persist and even grow. It may do so for extended periods, dissipating only when traffic flow decreases for long enough that more cars leave than join, slowly dissipating the mass. 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 “We know from watching cars take off at traffic lights there is a 1-second delay in cars starting” Last words past and present, plus questions, at last-word.com The new book out now: packed full of wit, knowledge and extraordinary discovery Available from booksellers and at newscientist.com/dolphin Will we ever speak dolphin? 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
Transcript of Pit problem
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.
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.
THE LAST WORD
Concertina carsI 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 non-linear and dependent on human
reaction times. If drivers had a reaction time of zero and could respond instantly to changes in the flow, an entire highway could start and stop together, like soldiers on parade. But we know from watching cars take off at traffic lights that there is roughly a 1-second delay between each car starting to move. It is probably no coincidence that the critical traffic flow that results in a complete standstill is about one vehicle per second.Hugh HuntTrinity CollegeUniversity of Cambridge, UK n While travelling towards southern England on the M5 motorway, I saw how a bunch-up in the traffic evolved, from start to finish. This was fascinating to me as a scientist.
Where it occurred, the motorway sloped down for about a mile, then levelled off. Just as I started down the slope, a lorry about a mile ahead indicated it wanted to move from the left-hand lane into the centre lane. This caused traffic behind to start bunching, presumably because drivers eased off the accelerator (I saw no brake lights). This bunching travelled backwards up the slope, like a wave.
When the lorry changed lanes, a second wave started moving up the slope, presumably when drivers braked (their brake lights came on). This second wave travelled faster than the first, merging with it eventually. Where the waves merged, traffic halted.
The merged wave brought more traffic to a halt as it moved up the slope past me and disappeared over the crest of the hill.
Once traffic had cleared the point where the lorry switched
lanes, it began to speed up and unbunched, creating a third “acceleration” wave. This, too, travelled up the hill, enabling stopped cars to move and quickly reach the speed they were moving at before the bunching occurred. This third wave also disappeared over the crest behind me.
For anybody who had not seen the lorry’s manoeuvre, or for anybody over the crest behind me, there would have been no apparent reason for the stoppage. Richard NolanCratloe, Clare, Ireland
n When a smoothly moving motorway is nearly at full capacity, glitches resulting from a driver changing lanes or braking will cause cars behind to brake, creating a mass of slow-moving or stopped cars. If the number of cars that joins the back of this mass matches or exceeds the number of cars leaving over the same period, the mass will persist and even grow. It may do so for extended periods, dissipating only when traffic flow decreases for long enough that more cars leave than join, slowly dissipating the mass.
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 KeshavSchool of Computer ScienceUniversity of WaterlooOntario, 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. JewettUniversity of California, San Francisco, US
“We know from watching cars take off at traffic lights there is a 1-second delay in cars starting”
Last words past and present, plus questions, at last-word.com
The new book out now: packed full of wit, knowledge and extraordinary discovery
Available from booksellers and at newscientist.com/dolphin
Will we ever speak dolphin?
This week’s questionPiT ProbLemMy doctor says that skin tags – small growths on the skin’s surface – commonly form in the armpit. Why do they appear?Ann GarminGlasgow, UK
121006_R_LastWord.indd 149 27/9/12 17:25:47
