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Experiment Shows Traffic 'Shock Waves' Cause Jams
Galactic_grub writes "Japanese researchers recently performed the first experimental demonstration of a phenomenon that causes a busy freeway to inexplicably grind to a halt. A team from Nagoya University in Japan had volunteers drive cars around a small circular track and monitored the way 'shockwaves' — caused when one driver brakes — are sent back to other cars, caused jams to occur. Drivers were asked to travel at 30 kmph but small fluctuations soon appeared, eventually causing several vehicles to stop completely. Understanding the phenomenon could help devise ways to avoid the problem. As one researcher comments: 'If they had set up an experiment with robots driving in a perfect circle, flow breakdown would not have occurred.'"
1. It's brakes. Brakes. Breaks is when something stops working. 2. This is obvious to anyone who has driven much. Try not to use your *brakes* on the motorway. Try to "iron out" the waves by ever so slowly dropping back when you see them approaching.
Get your own free personal location tracker
I have respect for my fellow drivers, and only use the gas pedal. Breaking is for pussies.
Well there's your problem right there.
You wouldn't have this problem if you wrote your own drivers.
If you can read this, I forgot to post anonymously.
It's already been done here, on Slashdot - already solved by the math guys, outlined on physorg.
But really any time I can see math at work in my day-to-day commute, is a good day to me. Also, it's fun to reach out and "touch" the asshole 200 yards behind you...
I knew it... I'm surrounded by *ssholes.
Keep braking, *ssholes!
TODO - Insert Creative/Witty Signature
You're missing the point of the experiment. Yes, in reality, something like that is incredibly likely. But the idea here is to study the effects other humans have on each other in dense driving situations.
Non impediti ratione cogitationus.
'If they had set up an experiment with robots driving in a perfect circle, flow breakdown would not have occurred.'
Is that true? If the robots had been fixed to a set driving speed (open loop), maybe. But if the robots had some sort of collision avoidance, it could still happen. It's instability in the control algorithm, no?
The M25 they mention isn't a perfect circle, which is a shame and most non-android drivers studiously ignore the speed restrictions until everything grinds to a halt. Its either 80 or nothing on the road and all the science in the world won't help.
If he's the Walrus then can I be a penguin please?
I can't find the reference, but clearly remember reading about the physics of traffic jams 20 years ago.
there are a lot of complex things going on, but two simple principles stand out
when someone ahead of you brakes, you need some time (distance) to react
if you are far enough away, you will slow the same amount as the person ahead of you
if you are to close to the vehicle ahead of you, then your reaction time is such that you will over compensate and over brake; the same to the person behind you and so forth
the trnasition between these two regimes is quite sharp
second, people slow for any distraction - a bright sign, a hill, whatever....
... as much as the next guy, but it's been done here many times. Slow news day I guess, but nobody is surprised by this. It's pretty much common sense.
See when you put cars in the article, that immediately takes away the ability to use a car analogy. No car analogies = no lively discussion, or something like that. It's an approximation. Adding Natalie Portman or something involving Ron Paul changes the equation slightly, but car analogies are where it's at.
I got a catholic block.
Then again, I remember seeing stuff like that back at the university, where they were trying to combine traffic models with a Kalman filter to achieve better traffic jam prediction. That was, uh, over five years ago.
...that cut people off, forcing them to brake. What makes this even worse here in Atlanta is the fact that nobody uses blinkers to indicate they are about to cut you off. I propose a system where cars of people who cut others off are immediately stalled. That'll help the traffic flow...
Its all the people driving SLOWLY that makes us aggressive people cut them off!
If they had set up an experiment with robots driving in a perfect circle, flow breakdown would not have occurred.
I, for one, welcome our new japanese robot driver overlords.
but seriously, I take this as a hint as to what is to come in the future for japan.
...spike
Ewwwwww, coconut...
Breaking or other external factors (an accident or flashing lights by the side of the road) can certainly precipitate a change from a swiftly moving flow to a slow moving flow. However, they only cause a transition when the density is high enough. If there's an accident during a low traffic time, you whiz by it. If they close two lanes out of four, and it's low traffic, you get a little backup, but it reaches a modest steady state size in low traffic. In high traffic you get a "wave" - the backup moves steadily backwards from the scene of an accident, and remains after the accident clears.
I often tell my wife that I can tell if a slowdown is just due to high volume or an accident by the abruptness of the slow down. An abrupt slowdown, I think, means heavy traffic "precipitated" into a jam by an external event.
So braking as described may be a precipitating event, but it's the sensitivity of the traffic flow to it that is the fundamental issue. I'd guess that even if people didn't brake so much, in those sensitive conditions a fender bender by the side of the road could cause a major backup.
(Clearly, I've thought about this WAY too much.)
It's not wasting time, I'm educating myself.
I have long thought that if there was a pair of LED's in the upper left corner of the vehicle, that indicated "at/above speed limit" or "below speed limit" this would solve many problems. The problem is that, like sound in gas, the notification to slow down is given by the car in front of you only (the molecule about to bump you).
But I could see a half a mile of cars all with little green lights, I could see (at the speed of light) the wave of yellow lights approaching and ease off the gas. The wave would be absorbed by this 'viscosity'. Traffic would flow near the speed limit or average flow rate, whichever the LED's were keyed to.
And don't even get me started on those GPS nav screens. Don't show me were I am. Show me where everyone else is. Let me see the compression 2 miles ahead and I'll chill (heh heh kinetic gas pun).
Guys, there really is a benefit to hitting the library and thumbing through back issues of ld technical journals.
I saw a History Channel Modern Marvels episode in highway tech and one researcher was using computer models and he determined it only takes one car to fuck things up for the rest of us. Let me repeat that it only takes one car driving slower than the rest of us to cause congestion and traffic jams on the highway.
"You'll get nothing, and you'll like it!"
Agreed, this is a classic feedback loop control problem, nothing really new at all, except that an electronic control system could easily iron out the resonances in this case.
http://en.wikipedia.org/wiki/Control_theory
In the human case the basic problem is with reaction time, a little worse than a tenth of a second.
Say a driver slows down for 5 seconds then returns to normal speed. The one right behind him has to slow down, but takes a tenth of a second to respond. The one behind him also has to slow down, now two tenths of a second later than the originator, and so on, by the time you get one hundred cars back the slowdown is 10 seconds behind the original event. The slowdown event in this case travels backwards like a wave at a rate of a tenth of a second per car "space", which is the car length plus the gap that the driver leaves in front of his car for safety.
If the slowdown wave travels backwards at the same speed that the cars are driving, you get one of those annoying events where everyone has to slow down at the same point on the highway (because the car in front had to slow down and so on), but there seems to be no reason for the holdup.
Being humans, the model is a poor approximation, some drivers might see the brake lights of a few cars ahead and react sooner, others are busy talking on cellphones and react later etc..
A reasonably well designed computerized driving system could easily remove the resonances, reacting far faster than humans, it would not be too difficult to design one that reacted in less than a millisecond or better.
I'm surprised the researchers seem to be discovering this now? Anyone bored and stuck in this kind of stop and go traffic on a freeway (and a little bit observant..) would have noticed this long ago.
Bavarian Purity Law of Rice Krispie Squares: Rice Krispies, Marshmallows, Butter, Vanilla.
I read a study prepared for Caltrans back in the 70's that deduced exactly the same thing. The state of traffic "science" seems to be about repeating the same insight over, and over, and over ...
Dog is my co-pilot.
Traffic Waves
Vivin Suresh Paliath
http://vivin.net
I like
1998 called and wants Its amazing news back Except he even built animated Gifs to illustrate!
meh
Perhaps Slashdot stories have a similar mathematical structure - even the dupes come in waves...
At the bottom of the
This phenomenon is obvious also to those who have gone through basic training. Often in marches you have the accordian effect where the front group is fine but due to small time differences in stopping and starting, that are amplified backwards, the rear unit often is running and stopping. The same occurs on the road whereby one person brakes and then many others brake before they need to. When traffic moves, people tend to start slower than needed. This is what creates these "phantom menaces" that backup traffic for no reason. Oh, that and people don't know how to merge correctly!
I really hope that this isn't truly a "new" discovery. If someone sits and thinks about it it should be really obvious. I have posted a basic explanation as a comment to a number of blogs and I'm not a traffic engineer.
Once traffic reaches a point of saturation ANY change is flow will bring traffic to a halt. This includes things like a road feature that makes people accelerate, a merging or diverging ramp, or a hill or corner that allows people to see farther ahead inducing them to adjust their speed.
Any change in relative speed will at some point cause a person to slow down. If they are accelerating they will slow when they get close to the car in front. If they are slowing then that is obviously a slow down. Each car behind this change reacts to it after a delay. The longer the delay the more they have to react. People tend to react much slower then they normally would because very few drivers focus primarily on driving. They only use the minimum attention that is necessary. This just makes the problem worse. It only takes a few cars for a subtle adjustment to become a complete stop. The length of time each successive car is stopped will become longer and longer.
To make the problem worse the same 1 to 2 second delay in reaction applies to the acceleration of the vehicles after the slowdown/stop. Every fraction of a second that a person delays accelerating is adding to the stacking of the delay.
To look at another aspect of a slowdown...
If you have a smooth flow of traffic at a fixed speed you will have a certain number of cars passing a point on the road per minute. If you have a slowdown you will reduce the number of cars passing per minute. But the traffic was already at a capacity flow so the number of cars feeding into this situation continues unabated. It is obvious that the flow of traffic is done until the quantity of vehicles feeding into the situation is drastically slowed to less then the flow of traffic at the restriction. (In Orange County, CA that means waiting till after 6:30PM for a situation that started at 3:00PM.)
What can be done about this without building a vastly increased number of lanes? (The I-405 is already 14 lanes in central Orange County. Increasing it to 20+ lanes would cost billions and in 20 years when they are done we would need 30+ lanes...)
A big improvement could be made through driver training (Yeah right...). Teaching people how to merge can reduce the constant forced slowdown from cars merging onto a freeway at less then freeway speed. The correct speed to merge at is 5 to 10 MPH FASTER then the flow of traffic. A car won't accelerate quickly but it will slowdown quickly so you simply drive down the on ramp picking the hole in traffic you will merge into and brake into that hole matching speeds. This minimizes the disruption. "Freeway Meters" on an onramp actually make this problem worse on a freeway that is still flowing at a reasonable speed because they reduce the distance that a car has to accelerate insuring that they enter the roadway at a reduced speed causing traffic to slow down for them.
Teaching people to slowdown after they are on an offramp will also help reduce the disruption to traffic flow. Most drivers slow down a minimum of 10mph before the exit the roadway causing large backups for open free flowing offramps.
To reduce the effect of slowdowns you can teach people to look past the car in front of them and try to slowdown before the car in front of them, which can reduce the quantity of speed they have to scrub off. If they look ahead and start to accelerate earlier this will cause a similar improvement in reduction of traffic impediment.
(Truth is that you will never be able to teach people to change their driving habits because their cell phone, their coffee, and their daydreams are way more important then driving the 1-1/2 tons of steel they are sitting in.)
Wow, I thought this was common knowledge already, at least within traffic engineer circles. In my little world, anyway here's a report from 1994...
Graduate students and most professors are no smarter than undergrads.
They're just older.
I played with this a couple of years back:
:-(
http://vwisb7.vkw.tu-dresden.de/~treiber/MicroApplet/
shame this post is buried down deep
Moral of the story: If you're afraid of driving, don't!
I was taking compressible fluid dynamics (that's the study of supersonic flow) when I picked up a book about traffic engineering. I noticed one of the formulas was very similar to a formula I was using lots in class. I looked more closely and realized that both equations must be describing similar phenomena. I've driven hundreds of thousands of miles since then, much of it in heavy traffic. A typical problem you solve in compressible fluids goes like this: there is air flowing in a pipe at over M=1 and there is a constriction in the flow. What is the velocity and pressure after the shock wave. It's a lot like traffic is moving at high speed and high density down a 4 lane highway and one lane is closed for repair.
Of course the analogy breaks down because... well it isn't the same phenomena. Each molecule of traffic has an emotional person controlling it.
Some of my observations:
When traffic reaches what I call critical flow (a combination of high density and high speed) then it doesn't take much disturbance at all to cause shocks (a shock being a rapid decrease in speed combined with an increase in traffic density, they are characterized by lots of brake lights.) When traffic is at critical flow, cause and effect can have a very non-linear relationship. Even a polite lane change, or a pothole might tip the traffic from critical to supercritical (traffic jam.) Sometimes shocks are standing shocks. This tends to happen approaching a constriction in the road when traffic sufficiently far back is sub critical. Sometimes shocks propagate backwards through traffic like waves at the beach. An excellent example of this is 880 Southbound approaching the turnoff to the San Mateo Bridge around commute time. This one is interesting because there is drag introduced by people exiting 880 but the main disturbance comes, I believe, from cars in the carpool lane cutting across all the lanes to exit to the San Mateo Bridge. As anyone who has driven this stretch of freeway can tell you, traffic blasts along at 70 then everyone is standing on their brakes, then 70, then brakes, then 70, then...
That was the best of my macro-traffic observations, here is one funny micro traffic observation. I call it the "sticky effect" or the "stupid effect" depending on my mood. If one car is overtaking another car but only going slightly faster the slower car will speed up at least temporarily to match speeds. If the passing car is going sufficiently faster then it won't happen. Two cars on a two lane road will frequently end up right next to each other as a result of the sticky effect. Causing a "moron roadblock," which is just a line of cars going too slow blocking every lane. I also call it a rolling roadblock, and when I lived in Utah I called it a mormon roadblock.
Drive safe!
-- QED
I'm convinced tailgating is the primary source of this problem. It's not so much that drivers want to tailgate but rather they're trying to drive at the maximum speed the conditions allow. This basically guarantees that they do tailgate and then are forced to brake on a regular basis to moderate their speed. And of course once anyone brakes it gets amplified because anyone behind them brakes even more. Aggravating this problem is the fact that many of these people are apparently incapable of coasting, and instead stomp on the gas every time they see an opening.
I've read, and have observed for myself, that big trucks act as dampers. Because they brake and accelerate more slowly they need to maintain greater distance from vehicles in front of them. They can nullify a traffic wave because by the time they've reached cars which had been stopped they've begun moving again which essentially ensures that the truck continues rolling.
I've applied this technique to rush hour traffic myself and unless traffic is particularly bad its extremely effective. Basically accelerate more slowly than the car ahead of me giving myself a considerable gap. And by considerably I mean a good 5 to 10 car gap. Then I let myself roll in first, maybe second gear. If I see cars braking ahead I regulate my speed more carefully. Most of the time, when I reach those cars ahead of me they're already accelerating again and I keep right on moving, maintaining a consistent speed.
What disrupts this is when idiots feel the need to get into any opening they see, worse, when they can't stand the fact that I've left an opening in front of me larger than they find acceptable. To them, they're not making progress if they aren't riding someone else's bumper.
I usually find that in rush hour this doesn't happen as frequently because people seem to be worn down an resigned to slow-moving traffic. They jockey for position a lot less frequently than they would on the weekend when heavy traffic is less common.
Then there's the New York area where drivers are overly aggressive and downright idiotic. There's nothing to be done then. But I also think their driving habits have arisen as a result of horrendously designed and constructed highways. I think better highway design could go a long way to alleviating traffic problems.
Depends on the design of the on and off ramps. Those off ramps that force you to slow down to 40km/h (25mph) about 15 meters after the off ramp stops... practically forcing you to slow beforehand. :(
Oh, and some locations have the cute idea to have an offramp and an on ramp in the same location, using the same stretch of road. Meaning you get on the freeway, and if you continue going straight you end up going off the freeway. This results in traffic trying to get on the road having to work around traffic trying to work around those that want on the road.
Design roads better!
I did a study back in the early '80s about traffic congestion in Los Angeles, CA and based the study on standing waves. It described how turns in the road, and other features actually contributed to the inefficiency of traffic flow. It also explained the bunching up of traffic in a wave pattern where there are actually areas mostly free of cars every few miles while other areas are packed up very tight.
This article is finding many of the same conclusions I had back then. Is there a fix? I don't know but traffic on a large scale is fluid.
God help us when we have flying cars and we have to deal with idiot drivers above us and below us!
Banjo - The more I know about Windoze, the more I love *nix
I agree with you somewhat, but there is another point of view on the merging problem, especially in heavy bumper-to-bumper traffic.
.02
Merging sooner increases the load in the merged-to lane and effectively lengthens its backup. And it wastes the available space in the merged-from lane. It also causes fluctuations in the merged-from lane as people speed up and slow down to accommodate the fluctuating availability of space. It's more efficient to use the "zipper" technique where *everyone* goes to the end of the merge lane and then zippers in to the lane alternately with those already in the lane. This makes it predictable for everyone involved. Those receiving the new cars know when to allow space and those merging in know they won't get stuck waiting for a gap.
But it only takes one idiot stopping early in the merge lane and forcing their way in to cause a large open space in front of them. Then the people behind him are looking at a large open space, accelerate into it. They effectively pass a bunch of people in the merged-to lane pissing them off by zooming by on the right and the whole thing falls apart.
man, I feel like mold.
You bring up an issue that connects with fuel consumption and safety on the road.
It is pretty rare that anyone truly questions the vehicles that we buy today. People have accepted SUVs as the norm for driving around in the US for some reason. People will recite a number of reasons but in reality the only real reason is that they have become accepted as normal and 'cool'. It is just a change in peoples perception. If you had asked someone in the 1970s to select a 'people hauler' they would select a station wagon. It is lower to the ground making it safer to drive and is a much better layout for a vehicle. No one would have accepted a 70s Suburban, Blazer, or Bronco (even with a more luxurious interior) because of the perception, "That's a truck!"
This change of perception is actively damaging our country. Moving the increased mass of these vehicles around just plain costs energy and increases wear and tear on our roadways. If you are conscious of this and want to get a reduced mass commuter vehicle you are taking your life in your hands because of the battering ram reality of a large percentage of the vehicles.
It is really time that we do something realistic about the mass of the vehicles on our roads. A general switch to smaller lightweight vehicles would massively reduce fuel expenditures, pollution, and the smaller size would help to reduce congestion. The solution to this is to change public perception, which will probably only come about by economic reasons. The price of gas will do part of it but taxing vehicles by weight will go a long way towards making it a more equitable situation. The lighter vehicles would be rewarded for their lessened impact on the roadway and the environment. More massive vehicles would pay for increased impact on the environment, wear to the roadway, and the increased risk they pose to lighter vehicles.
People talk about rail/mass transit as a solution to LA's transportation problems but usually it is people who haven't been here. This area is so spread out in all directions that it would take an indescribable quantity of money to build such a system. The land is insanely expensive and with the sprawling area you could spend a few years of our whole nation's federal budget to build such a system. It just won't work.
(That guy who stole the tank down in San Diego about 5 years ago is starting to look saner and saner...)
BTW - From trips to Europe: I love the mass transit! Munich has a rail, subway, tram, and bus system that is so good it is just fun to use it! For 15 euro you get a pass good for unlimited use of all of the above for a week! It is just amazingly convenient and cool.
Even ambulances with sirens and lights operating don't drive that fast, because the risk of seriously hurting or killing someone else is too great compared to the benefit gained from time saved. No, ambulances with lights and sirens don't drive that fast because they recognize that they're driving very slow heavy diesel trucks not meant to accelerate, brake, OR handle, and that most of their time savings therefore come from safely negotiating intersections without stopping and waiting in traffic. If they have to spend a long time on the highway there is a problem anyway, as their base is too far from the people they need to serve.
Ambulance drivers are smart enough to realize it's unsafe to drive an AMBULANCE as fast as a car, or even most of the SUVs that soccer moms are ignorantly piloting.
And, sorry to pull out the autobahn again, but seriously, the speed itself isn't that unsafe. I'm not saying I should be able to drive 120mph thru traffic in the US because I realize myself and my roadgoing counterparts are not as well trained, and people simply aren't expecting to see that here.
But it's simply unintelligent to say that you can't drive quickly with reasonable safety.
I'm sorry, but I think the OP can figure out whether a
Criticize people for causing accidents. Give them fines, even jail time if they were truly being reckless. But don't tell me my speed alone makes me 20x as likely to be in an accident.