Crosstown Traffic | Cocktail Party Physics Scientific American…

17 Июн 2015 | Author: | No comments yet »
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Crosstown Traffic

Jennifer is a science writer who loves to her inner geek by finding connections between physics, culture, and the world at large. on Twitter @JenLucPiquant .

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In the 1993 Falling Down . Michael plays William Foster, D-FENS, a recently divorced who has been laid off from his job a defense agency. We first him when he’s on his way to his daughter’s party on a hot summer day, in a traffic jam on a Los Angeles freeway no air conditioning and an errant fly buzzing the baking interior.  The stress until Foster snaps, his car on the freeway and heading off on foot, going on a violent rampage the city as he unleashes all his anger and at being a forgotten man.

road rage, writ and while we certainly can’t Foster’s behavior, who among us felt that slow of frustration and impotent rage trapped in a seemingly infinite of cars creeping along the I spent two years commuting the east side to the west of Los Angeles, and I assure you that Ninth Circle of Hell is stretch of I-10 between and Robertson Boulevard at 8:30 AM other commuters might to the I-405).

Nor is this merely a Los problem, although the city of frequently tops the list of with the worst traffic in the along with Washington, DC, San New York City, Atlanta, and even Seattle. Traffic is a big problem in most major centers. The average US motorist 36 hours every year in delays.

Fortunately, physicists are hot on the and have been busily traffic patterns and trying to mathematical models to devise kind of solution to keep us all snapping and turning into

Going With the Flow

scientific wisdom has compared jams to the process of freezing — a phase transition between a to a solid. Think of it this On a sparsely populated highway the are far apart and can move at whatever they choose while maneuvering between lanes — like the movement of molecules in a In heavier traffic, the “car are more densely packed, less room to maneuver, so move at slower average and traffic behaves more a liquid.

If the “car molecules” become too packed, their speed is and their range of movement is to such an extent that can “crystallize” into a solid. So jams aren’t random. a threshold “value” to the flux of traveling along a highway, and if threshold is exceeded sufficiently — if perturbations are large enough — the flowing “liquid” traffic into a “solid,” akin to the temperature/pressure point threshold water turns into

It’s a useful but rather analogy. The situation is a bit more than that, and scientists love to understand the phenomenon in far detail. A major breakthrough in 1998, when a physicist Boris Kerner with the Benz Research Institute in Germany, published a paper in Review Letters . (Kerner has an entire book on the subject. a highly technical tome The Physics of Traffic: Empirical Pattern Features, Engineering and Theory. ) Kerner analyzed collected from several of traffic monitored along highways and found that tends to follow the physics of organization.

Based on that he developed a model that broke traffic into basic categories: freely jammed (solid state), and a intermediate state called flow, in which densely “car molecules” move in like members of a marching When this happens — all the cars are traveling at close to the average speed because of the density on the roadway — they highly dependent on one another. A might compare the relationship to the motion of electrons in metals, gives rise to weird like superconductivity.

Highly traffic means that a perturbation — a butterfly flapping its or a single driver braking — will send little of corresponding slowdowns through the chain of cars behind That’s one reason why slowdowns and jams occur most at merge points, especially and entrance ramps, or when are closed due to road construction.

A of steady synchronized flow, by these tiny ripple (“narrow jams”) can persist but the balance is delicate and highly If the volume of cars continues to the density continues to increase, and you get a “pinch effect” — that “stop and go” phenomenon, in which you one narrow jam only to encounter a little further down toe until they all converge a single wide jam. comes to a standstill. Collective rage may ensue, and the next you know, Michael Douglas is on a rampage.

Kerner’s self-organizing model seem to indicate that drivers think they are of their own accord, they are just behaving like a unthinking particle, a single of sand in an enormous sand unconsciously following the “rules” of flow.

But that’s not quite the driver behavior can impact patterns in significant ways. The of human behavior is one reason why traffic models aren’t as in their predictions as they be.

In 2004, a team of German led by Michael Schreckenberg (University of sought to rectify that. came up with new predictive that took realistic behavior into account: you all those jerks on the highway who changing lanes and cutting in of you just when your is starting to move, which in arouses your competitive so that you (a) honk angrily and him for a few yards, determined not to let any other infringe on your valuable or (b) switch over to a new lane in thereby causing another Not that we’re bitter. As member Robert Barlovic put it, drivers tend to hinder other when doing like changing lanes.”

The was quickly put into use to forecast along the autobahn network Cologne, based on real-time data gathered by  embedded in the road. The problem is that broad access to more information actually changed patterns. People modified behavior based on the new information by the forecast models: they all to the same exits to avoid congestion, for example. This in made those models accurate in their predictions. information is not, as it turns the answer to all our traffic woes.

cities are trying similar In May, San Francisco started up its Traveler Research Initiative. combines real-time traffic with databases of past patterns — information already by numerous companies like and Google — to predict jams “up to 40 into the future. Drivers are automatically sent an email or message of conditions on their commute before their begins.” That way they can their routes and avoid the areas. Only a dozen or so are currently enrolled in this program, and MIT engineer Moshe told New Scientist that all drivers have access, likely get the same flocking as in Cologne, Germany. “You them of congestion in one location, and the just shifts to another

Phantom Traffic Jams

maddening of all is that there always seem to any good for the congestion. Granted, it’s heavier near an exit but more often than you creep along, expecting to some god-awful wreck… and nothing. Traffic suddenly to pick up just as mysteriously as it down.  In 2008, a team of scientists at Nagoya University to the somewhat obvious conclusion there are just too many on the road. It’s a density There’s a certain critical for traffic, and once it’s even tiny fluctuations can a chain reaction that results in a jam.

They their theory by studying 22 driving around a circular asking the drivers to move at a 19 MPH. Twenty-two cars was the number to achieve critical on the track. The drivers did their to maintain the requested speed, but were still tiny of braking and speeding up, and this around the track. The result: brief standstills. Get enough of over the course of rush on a weekday morning (or evening), and you’ll get a traffic jam.

A of MIT mathematicians were so intrigued by the researchers’ findings that developed their own models for kinds of phantom traffic (or “jamitons”), publishing their in Physical Review E in May 2009. equations are very similar to the used to describe the shock produced by explosions, adding variables such as traffic and car density to calculus the precise these “jamitons” are most to form. Just as with waves, jamitons have a of “sonic point” that the flow of cars into and “downstream” segments. Those can’t communicate with That’s why, if you’re in a car “downstream,” you have no way of knowing if an accident or some other impeding traffic flow “upstream” — or if conditions are about to as suddenly improve.

Gabor of the University of Exeter is a mathematician who has concluded that “the of spontaneous jam formation (caused by fluctuations above a critical density) is the main reason for jams.” He has studied the reaction delay of drivers and found a late reaction of just one by a single driver can have a impact, particularly at faster A vehicle dropping its speed 80 MPH to 65 MPH may cause a ripple that vanishes, while dropping its from 80 MPH to 62 MPH may cause a ripple is amplified and leads to traffic .

So by now, physicists and mathematicians successfully defined the problem of traffic jams. Alas, a has proved more difficult to by.

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The Ants Go Marching One by One

Perhaps we can a few lessons from the Formicidae a.k.a. the humble ant. A appeared on the arXiv in October demonstrating that ants be better than humans at their own traffic efficiently. Helbing is a “congestion expert” at University of Technology, who teamed up a few colleagues to build a tiny ant in the lab, featuring several between an ant nest and a source of It didn’t take long for a few to find the shortest route to the leaving a handy chemical for their friends to follow. And so many ants were that trail it became with ants.

Now, if happened on the I-10 (or a tiny ant of it), it would be the point of density, and the tiny fluctuations in behavior would build up and traffic jams, especially at the But that’s not what happened the ants. At those critical just when the route was to become too clogged, ants to the nest would physically the way for ants on their way to the sugar Not consciously, mind you — there wasn’t enough room. The ants had to find an alternate

As result, traffic jams formed. Somehow the humble ant has on of the most challenging problems in physics, not to mention the routing of over the Internet and other systems: “the efficient of limited resources by decentralized, decisions.”

Helbing realizes it’s not practical to let cars with oncoming cars to traffic, but he figures that you can cars traveling in one direction to oncoming cars to the traffic ahead, so they can take action if they need to. The with this, as mentioned is that the evasive actions can their own clogged conditions.

So why not more roads, or widen the to accommodate more cars? It out this might not be an optimal either, according to a paper in Review Letters in August (2008 was a banner year for research). A collaboration of Korean and US conducted a study and concluded building more roads can make traffic congestion It’s a paradox, stemming the fact that individual act solely in their own best — say, the quickest route for — and instead end up slowing traffic as a

How many times have you someone drive along the or an exit lane during traffic to sneak a few spots in line, thereby slowing the merging to nearly a standstill? If cut in line in the grocery store, would be shamed into at feeling a twinge of guilt it, and I’ve seen cashiers toss those customers to the of the line in retaliation for their But in a car, you have the protection of — mobile privacy — and the offender stick around long for the social peer pressure to in.

It’s basically a conflict of between individual and collective — the researchers dubbed it “the of Anarchy” — and the result is 30% longer times overall. How do we change nature, which seems to act to the collective best interest it comes to traffic? Well, you can try to a change in behavior, like the The Korean/US team found by simply shutting down a few streets, they eliminated travel options, thereby the interests of single drivers in line with the interests of commuters as a whole.

Here’s illustration. Select starting and points linked by two different one over a short but narrow and a longer one over a wide If half of the drivers choose the and the other half choose the the combined travel for all is minimized — many more drivers opt for the route over the narrow which quickly becomes When it gets clogged some of the drivers switch to the highway.

This process itself out over time the traffic reaches some of equilibrium, a state in which no one can reduce their commute by switching to the other one. But the total commute time is longer even after all switching than if the drivers had split down the middle each route to begin Shutting down carefully streets can encourage drivers to the optimal choices — without they are doing so.

Once again, the ants out to be smarter than humans, at when it comes to optimizing for the good of the collective whole. biologist Vincet Fourcassie Sabatier University in Narbonne) set up his own two-way ant highway with a of varying width between the and a foraging site to study the of both garden and leaf-cutting And he observed very definite of the road” that came play — the ant equivalent to not cutting in at the supermarket. For instance, when the was narrowed so that only one ant could pass, an ant can’t the bridge if another ant is moving it in the opposite direction, but the same ant can another ant directly in front of it. ants have more concerns: here, if an ant is carrying a (food) back to the nest, ant receives right of way on the bridge; the ants will yield, refuse to pass the ant bearing

Basically, ants are better at their behavior for the common They even will use tiny bodies to plug in the trail leading back to the for faster, more efficient of food. Human beings, in constantly struggle with the between the common good and in it for me?” We could learn a few from the ants.

[ NOTE . post was adapted from two blog posts from the Party Physics archives: Traffic (January 2009) and Rage Redux (August .]

About the Author: Jennifer is a science writer who loves to her inner geek by finding connections between physics, culture, and the world at large. on Twitter @JenLucPiquant .

car Smart Crosstown
car Smart Crosstown
car Smart Crosstown
car Smart Crosstown
car Smart Crosstown


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