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.'"

Brakes. Not breaks.

[caluml]

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.

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[account_deleted]

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I love discussing traffic jams...

[explosivejared]

... as much as the next guy, but it's been done here [slashdot.org] 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.

Re:physorg

[Lumpy]

Yup and researched and discovered more than 5 years ago. Unless there is something vastly different from Japanese drivers compared to the drivers here in the usa it's simply confirmation that the research done by grad students here is 100% correct.

Anyone that has paid attention and driven in heavy city traffic has seen this. The hill coming into detroit on I96 you can watch in the early morning a wave of breaklights coming to you from a mile away. the undulation continues for the next 30 miles and probably lasts for most of the commute times.

Re:prehistoric

[foobsr]

I can't find the reference, but clearly remember reading about the physics of traffic jams 20 years ago.

I do as well, and I recall there was even software (e.g. GPSS) to simulate the phenomenon. But nice to see how an experiment validates historic findings (which have probably not made it to Google yet and thus practically do not exist).

CC.

1998 called!

[bigattichouse]

1998 called and wants Its amazing news back [eskimo.com] Except he even built animated Gifs to illustrate!

Obvious

[magicalyak]

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!

Maybe the first experiment, but hardly new

[theonlyaether]

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... [newscientist.com]

Re:Brakes. Not breaks.

[Critical Facilities]

the jerkoff Sunday Driver creeping along at 50 in the passing lane just has to be dealt with,

I could not agree more. These people seem to take pleasure in being a complete obstacle. For the life of me, I cannot understand why someone would willingly drive slowly in the left lane it's insane and definitely a major contributing factor to this problem.

If police would enforce rules against driving too slowly (generally defined as being passed on the right ...as they do aggressive driving, the problem would be much less prevalent.

While I agree, and I would like to see that enforced better, we should be careful about what we wish for. I just recently got an education (from an area police officer with ticket book in hand....$375 later) that passing on the right is ALSO illegal. See various links below:
http://www.nysdmv.com/dmanual/chapter06-manual.htm [nysdmv.com]
http://search.dmv.org/dmv/passing%20on%20the%20right [dmv.org]
http://www.dmv.state.va.us/webdoc/pdf/dmv115.pdf [state.va.us]
http://www.onlinetrafficschoolguide.com/me-maine/driving_laws.html [onlinetraf...lguide.com]
etc.

Re:Not that simple

[cbiltcliffe]

I really hope that this isn't truly a "new" discovery.

It's not. This guy [eskimo.com] was an amateur looking at the problem a decade ago.

Re:stability

[teslar]

It probably has to do with reaction time. If the robots maintain perfect velocity synchronicity with the car in front, you'd probably not see the wave propagation. On the other hand, if the robots were configured to have a response "delay" ( on the order of, say, half a second or so ) and minor errors in estimated velocity correction you'd probably see something just like you'd get with humans.

Doesn't work that way. Response delays are a given, you don't have to configure them in explicitly. Signal comes in from the sensors, has to travel a certain length of wire - this takes time. Signal has to be processed, response computed - takes time. Response has to be communicated to whatever reaction mechanisms you have (throttle, brakes...) - takes time. Mechanism has to be activated - takes time. etc. Granted, everything is on the millisecond scale, but there is no such thing as a zero-delay response and thus no such thing as perfect velocity synchronicity.

Ditto with "estimating" velocity correction or even just velocity and the velocity of the guy ahead - you don't have to explicitly introduce errors, they're already part of the system (we wouldn't call it an estimate otherwise). Heck, even making sure that you're actually travelling at the velocity you think you're travelling at is not all that easy - there are a lot of mechanical parts between you and the wheels on the road, they'll introduce fluctuations (including the wheels and the road themselves) - so just because you figured that injecting 5mg more petrol for the next 62.54235ms will make you reach your optimal speed of exactly 30km/h, that don't make it so; there will always be a small error.

Point being, there is no such thing as perfection in the real world and I would advise you never to expect that of a robot or other device. Errors may be small and therefore neglectable, but they exist.

Re:Brakes. Not breaks.

[omgwtfroflbbqwasd]

For the life of me, I cannot understand why someone would willingly drive slowly in the left lane..

For years this boggled me as well, but then I figured it out. Many lazy drivers don't like to change lanes and they don't like other drivers merging in front of them. Perhaps it distracts them from their cell phones or whatever else, but people like to pick a lane and camp in it.

With the 6-10 lane mega-freeways, the traffic merging in and out of the freeway (from the right) cuts through the rightmost and middle lanes. Anyone who's in the leftmost lane doesn't have to "worry" about traffic merging into or through them.

Re:Not that simple

[jlp2097]

I think what's new here is that it's been shown in an actual experiment using real cars, rather than just theorized or modeled in a computer.

This has been known and studied for a long time in theory and practice. I remember seeing pretty much the exact same experiment on a pseudo/popular-science TV show in germany, called "Welt der Wunder" about two years ago. Proof [weltderwunder.de]. Even though the link is in German, there is a date (January 2007) and the picture at the top right shows the experiment. They also mention that there were no problems using 8 to 10 drivers at 30 km per hour, but adding more drivers made these waves occur until everybody slowly stopped. So I don't see how this is newsworthy...

Practical Experiments? MC is cheaper and better.

[twitter]

Physical experiments have their limits too. What's practical about drivers running around in circles? Real roads have fluctuating traffic loads, blind turns, merging and diverging traffic and a host of other obstacles. The point of an experiment is controlled conditions to gain fundamental constants and other descriptors. What you might get from this experiment is better statistics on driver reaction time and a few other constants to refine you models.

When you really want to know how a road is going to perform, you take it to some kind of Monte Carlo simulation.

Re:Not that simple

[Scottie-Z]

I've seen this in a textbook [ec-securehost.com] published in 1998, which contains references to papers dating from the 1960's. That text highly recommendeds a collection of review articles found here [amazon.com], and what appears to be a later book by the same author is here [amazon.com].

Re:Not that simple

[lena_10326]

What happens if there is a break down? You are back to three lanes of traffic, just like there would have been if you had never allowed driving on the hard shoulder in the first place. The rest of the time, you get an extra lane.

You're not considering safety. Someone broken down on the side of the road most likely will be walking around the car, possibly changing a tire, making a phone call, or walking to the off-ramp. Traffic in the shoulder is a serious danger to that person. It would also make it more difficult for the tow-truck to get to and pickup the disabled car because any gap room that would have been there is filled with cars. It would also make it a lot more difficult for ambulances and police cars to get to the accident, which is probably causing the traffic jam in the first place.

Re:Not that simple

[LakeSolon]

I really hope that this isn't truly a "new" discovery.

It's not. This guy [eskimo.com] was an amateur looking at the problem a decade ago.

Geez, not THAT guy again. His observations on what causes that stuff are spot on, but his proposed solutions show a complete inability to understand the concept of scaling as it applies to traffic. He notes that by keeping a larger interval in front of him, the "wave" disappears. Well no shit. Doing that simulates a small pocket of uncongested freeway. This pocket is created at the (small) expense of the cars behind him. You can't have everyone leave a larger interval because that would require the road to be carrying fewer cars. The waves are caused by too many cars too close together. no amount of driving "tricks" is going to increase the car-to-car interval without actually reducing traffic density.

Commendable effort, but it's further proof of what my father (an engineer) has always said about engineers "Never ask an engineer to solve a problem outside his area of expertise. You'll get the most plausible sounding wrong answer you've ever heard."

To respond to your specific claim:

You can't have everyone leave a larger interval because that would require the road to be carrying fewer cars.

The goal is not traffic density (cars per mile of roadway, f'ex) but rather traffic throughput (cars per hour).

If you double following distance you reduce density by half*. If you were to continue at the same speed you'd also cut throughput by half. But if the extra following distance avoids propagating perturbations that would cause slowdowns your average speed may well more than double thereby increasing throughput.

And it may not, but your claim is insufficient to show increased following distance is counterproductive to throughput (never mind safety concerns).

* Since horses are all frictionless spheres, naturally cars must have zero length.

P.S. The linked site is truly one of the classics of the internet. I believe it's been posted on slashdot before. And then presumably duped a couple times for good measure.

Re:Not that simple

[Anonymous Coward]

Down here in New Zealand one of the worst causes of jams is due to the motorways going from 2 lanes to 3 then to 4, then 3, then 2 then 3 and so on in the space of a few miles. Traffic merging from one lane to another causes it to to back up to the previous on-ramp which then cascades till the speedo hits 0 and hardly moves anywhere

I can see the same happening with using the hard shoulder as a lane - car breaks down and pulls over, then the cars behind it have to merge into the next lane slowing the traffic worse than it would have been if there were only 3 lanes and a hard shoulder.