Cracking the physics of traffic jams
If you sit in a some gridlock this holiday season, you might come to the end of the line of cars and realize, hey, there's nothing there. No accident, no police on the shoulder, just a bunch of cars that aren't getting where they want to go. Over at the Universities of Exeter (in England), Bristol and Budapest, mathematicians now think they've figured out why this happens (and wastes lots of gasoline in the process).
The short answer: braking and full roads. When there are between 10 and 15 vehicles on a one-kilometer stretch of highway and the front one hits the brakes, a "backward travelling wave" is created that can sometimes lead to traffic jams. As Dr. Gábor Orosz of the University of Exeter's School of Engineering, Computing and Mathematics, said in a statement: "As many of us prepare to travel long distances to see family and friends over Christmas, we're likely to experience the frustration of getting stuck in a traffic jam that seems to have no cause. Our model shows that overreaction of a single driver can have enormous impact on the rest of the traffic, leading to massive delays." He continued: "When you tap your brake, the traffic may come to a full stand-still several miles behind you. It really matters how hard you brake - a slight braking from a driver who has identified a problem early will allow the traffic flow to remain smooth. Heavier braking, usually caused by a driver reacting late to a problem, can affect traffic flow for many miles."
This seems like a problem with no solution. Not braking could lead to accidents, which certainly don't make the highways easier to travel on. And removing cars from the road would be appreciated by many, until public transportation becomes a better option, it ain't gonna happen. So, if you get stuck on the way to or from grandmother's house this year, at least you now kind of know why.
[Source: University of Exeter via Scientific American]
Reader Comments (Page 1 of 1)
Matt Hopley 11:54AM (11/08/2008)
agree with ernie there on that one :-) its pretty fun seeing how just one driver can influence miles of traffic behind him by just keeping things moving. the key i guess is to keep as much distance as possible and to avoid the stop starting method that most drivers adopt.... traffic is pretty complicated though and modelling it isnt too easy... here's my attempt of comparing it to granular physics and even chaos theory... have a read www.cactusconnects.co.uk/physics_of_traffic_jams
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Matt Hopley 11:55AM (11/08/2008)
forgot to add the full link with the http.... here it is again, http://www.cactusconnects.co.uk/physics_of_traffic_jams.htm
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mattypape 8:54PM (12/25/2007)
Well, havent we all known this for years? this is like the 3rd site ive seen this story on this week and its nothing new, just a new computer model which simulates what people have said for years.
And we have all heard how motorway speed limits are dropped to 50mph during heavy traffic as it actually increase the average speed.
and i just looked on that source sites comments and its called the 'accordion effect' and was devised 40 years ago.
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Ernie 12:31PM (12/30/2007)
Um, yes, this is old news. The solution is not to brake so damn hard in the first place. If you try to keep things moving (even just crawling along) then those waves stop and traffic flow picks up again. And again, all it takes is a single driver to fix the problem. Instead of accelerating as hard a you can and braking as hard as you can, just keep creeping forward for as long as you can. It's just that simple.
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Benjamin Jones 1:52AM (12/26/2007)
Interestingly enough, this was just discussed here: http://forum.ecomodder.com/showthread.php?t=309
To sum up my opinion on it, I see it like this: from a pure ecodriving standpoint, I often find myself (in a 91 crx) getting better fuel economy in avg 20mph traffic, as I can use the slow pulse and glide nature of the beast to really stretch out some low speed coasts...I once got 70 mpg in a really stagnant bit of traffic where the rest of my trip at 55 mph only netted me ~60 mpg...
However, I guess the final solution would just be some damn public transit, eh?
-------
Benjamin Jones
http://www.ecomodder.com
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Nucbuddy 7:03AM (12/26/2007)
"When [...] the front one hits the brakes, a "backward travelling wave" is created"
Tom Cruise mentioned this in Mission Impossible 3.
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Rich 7:47AM (12/26/2007)
With all the traffic cams out there - the driver causing the problem should be removed from the road and given the option of driver education or no driver's license.
I've seen this type of problem all my life and it's very frustrating because it's usually completely avoidable and being caused by someone who is clueless about the results of their actions.
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Cody 9:23AM (12/26/2007)
Just remove all drives.Humans faults will never be over come. The sooner computers do the driving the better.
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Throwback 9:58AM (12/26/2007)
When computers start doing the driving people will go back to horses. While I'm sure a car computer would never go down, just imagine if their was a computer failure on the highway. Who is at fault then? Not the driver (I wasn't driving), not the computer (it's a insert favorite system) they are very reliable. It must be the government's fault for not mandating perfect computers and traffic systems. We simply have too many more cars on our roads. Our population is increasing and with our immigration trends, both legal and illegal, that won't change anytime soon. The solution is to drive less if possible, drive during off hours if possible, take public transportation where possible, or suck it up and just be mad while in traffic jams.
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David Mustoe 10:18AM (12/26/2007)
The original work was done with piano wire on reels strung between test cars and reported in Scientific American.
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Steve 10:50AM (12/26/2007)
A problem with no solution? Please. First of all, step up tailgating enforcement. I say HUGE tickets for anyone found to be within 20 feet of the vehicle in front of them. Furthermore, we should have cameras watching the roads, not for speeding, but for the people who cause that backward wave. Those people should LOSE THEIR LICENSE and face the same no-license period as someone caught driving without a license.
The other problem in traffic jams, which keeps them from simply being "slow down, stop, and then speed back up" events, is people who decide that they NEED TO BE IN THE OTHER LANE, even though that lane is moving, while theirs is stopped.
We have the technology to start enforcing against the causes of traffic jams, which boil down to tailgating, driver inattention, and impatience. Cars should have a button you can push to send a photo of the license plate of a tailgating car to the police, and that driver should be issued a ticket automatically (the speed of travel would have to be recorded as well, and the ticket scaled according to speed, with a lower limit at, say, 35-45 mph).
Eliminating traffic jams would probably reduce our emissions/fuel usage by a huge amount. There's no reason not to do it. No one likes traffic jams, and they cost us all huge amounts of money in fuel and maintenace (that idling takes a toll on the ol' engine) costs.
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toast2042 6:16PM (12/26/2007)
to fix this we only need to allow enough room between one car and the next. the one-car-length-per-10-mph rule would fix this easily.
that, and shock therapy for the selfish morons who somehow get licenses.
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MikeW 4:21PM (12/26/2007)
The problem is the lack of positive signalling brake lights.
Brake lights should be off-ON only.
None of this penny pinching tail/brake BS.
Since not many cars have this, too many [old] people are speculative brakers, overbrakers. also bad technique-don't look at the car in front of you, look at the car two in front of you.
I take pride in stopping the backward propogating wave. (I know how to downshift an automatic with the manumatic interface)
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MikeW 1:05PM (12/27/2007)
No, more like 0.5 seconds per 25mph. [you can basically drive asleep like this-and some people effectively are]
25mph-18.3 feet
50mph-73.3 feet
75mph-165 feet
100mph-293.3
125mph-458.3
150mph-660
175mph-898.3
200mph-1173.3
225mph-1485
250mph-1833.3
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DC 3:07PM (12/27/2007)
Mike 0.5 seconds per 25 mph and 1 car length per 10 mph achieve similar results.
25 mph = 18.3 feet
50 mph = 73.3 feet
75 mph = 165 feet
Assuming 15 foot car lengths
25 mph = 2.5 car lengths = 37.5 feet
50 mph = 5 car lengths = 75 feet
75 mph = 7.5 car lengths = 112.5
You're suggesting that at 25 mph one needs to be barely one car length behind the next person? I think I'll take the 2.5 car lengths and actually be able to negate some of the traffic issues discussed above when that person hits their brakes thanks.
1 car length per 10 mph stands IMO, it makes sense for emergency stopping as well considering most vehicles stop from 60 mph in under 150 ft, some closer to 100. That means at 60 mph giving the car ahead 6 lengths = 90 ft, you wouldn't rear end them anything short of a cement wall being placed on the road.
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PJD 10:54PM (12/27/2007)
Perhaps, sensors like are being used in the new cruise control systems could be used to detect when a car is closing the gap with a slowing car ahead and give a warning light to those behind even if the driver has yet to notice or started braking.
I've known too many, mostly young male, drivers who think it's showing off their "great" driving ability to wait until the last second to brake. They may indeed be skilled and attentive enough to drive like this, but they fail to see that not everyone behind them will be the same. It leads to these traffic jams and even having a less attentive driver behind slam into said "great" driver.
A proximity sensor system as suggested above could provide a more uniform message to those driving behind. It's in effect looking two cars down the road but doesn't require one to have visibility, which one often doesn't if stuck behind a much larger vehicle.
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mkM3 6:28AM (12/29/2007)
Brakes are primarily for stopping. There are no intersections or stoplights on a freeway, so brakes are only truly necessary in rare cases. As such, I typically don't use my brakes on the highway. Usually I just use a foot-lift to slow down or sometimes a downshift and foot-lift. The only times I need the brakes are if someone cuts me off or if someone is tailgating me.
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