Cocktail Party Physics

crosstown traffic

Let me once again offer a mea culpa for lack of blog activity. I can't speak for my fellow bloggers, but I have been stuck in traffic, which really cuts down on blogging time. And Jen-Luc Piquant won't let me blog and drive. I can't even Twitter and drive, since as of January 1, it is illegal to text while driving. (Seriously? They had to pass a law on that?) It's about the only downside to my new job, after years of having the world's shortest commute, i.e., the distance from the bedroom to the office. Now I'm inching my way forward on the I-10 twice a day with all the other poor commuting sods, guarding my precious freeway real estate from the invading hordes of merging vehicles. (Listening to soothing classical music helps. I might even start practicing my French again. But how I miss NYC subways, where I could get all my reading done during the morning and evening commutes.)

I've experimented with alternate routes. The problem with that strategy is everyone else has the same idea, and in no time at all those routes become just as jam-packed. But have no fear! The physicists are here! Modeling traffic patterns is a very active area of physics research; the tricky part is taking into account irrational human behavior. For instance, in 2006 I wrote about the model devised by a team of German scientists at the University of Duisberg, which was used to forecast traffic along the autobahn network in Cologne. The data for the model was collected by embedded sensors in the road, so any German driver who wanted it could have access to the traffic forecasts for that particular stretch up to an hour ahead of their commute.

But more and better information did not help improve traffic flow. Access to that new information actually changed the traffic pattern, and not necessarily for the better. I have witnessed this phenomenon firsthand. Give us a heads-up on potential problems on our route home, and we will all flock like panicked lemmings to the same exits and alternate routes, desperate to avoid congestion, and thereby creating new points of congestion. It's a vicious cycle, I tell you. Dante's Ninth Circle of Hell is that stretch of I-10 between Crenshaw and Robertson Boulevard at 8:45 AM.

Most maddening of all is that there doesn't always seem to any good reason for the congestion. Granted, it's always heavier near an exit ramp, but more often than not, you creep along, expecting to pass some god-awful wreck... and there's nothing. Traffic suddenly starts to pick up just as mysteriously as it slowed down. You'd think there would at least be a spot of road construction closing down one lane -- just so you know you've suffered for a reason.

Last year a team of Japanese scientists at Nagoya University reported on their traffic work spanning a decade, concluding that the problem is quite simple, really: there are just too many cars on the road. It's a density problem. There's a certain critical threshold for traffic, and once it's reached, even tiny fluctuations can cause a chain reaction that eventually results in a jam. You know, like when a law-flouting driver in the Audi ahead of you decides to text his girlfriend and then has to brake too suddenly when he looks up and realizes he's about to rear-end the BMW just head. That makes YOU brake too suddenly, and the person behind you, and the person behind them, and so on. (Even if the end result isn't a traffic jam, all that jerky stopping and starting is hell on the old gas mileage.)

Like any good science team, they tested their theory by studying 22 cars driving around a circular track, asking the drivers to move at a steady 19 MPH. Twenty-two cars, apparently, was the magic number to achieve critical density on the track. The drivers did their best to maintain the requested speed, but there were still tiny fluctuations of braking and speeding up, and this reverberated around the track. The result: occasional brief standstills. Get enough of those over the course of rush hour on a weekday, and eventually you'll get a traffic jam.

The Nagoya researchers aren't the only ones who've explored this problem. Gabor Orosz of the University of Exeter is a mathematician who also concludes that "the effect of spontaneous jam formation (caused by tiny fluctuations above a critical traffic density) is the main reason for traffic jams." His studies also include the reaction time delay of drivers -- turns out that a late reaction of just one second by a single driver can have a major impact, particularly at faster speeds. "A vehicle dropping its speed from 80 MPH to 65 MPH may cause a ripple that later vanishes, while dropping its speed from 80 MPH to 62 MPH may cause a ripple that is amplified and leads to traffic jams," he said in an article in the Telegraph last March. So just because there's heavy traffic on the freeway doesn't necessarily mean it can't flow freely. There are lots of factors involved.

Nor are human beings particularly inclined to seek out alternate routes; in general, we prefer the familiar ones. Last February, scientists at Utrecht University reported on the results from an experiment involving 72 people, who were asked to complete surveys in one room, then taken to another to complete the experiment, via one of two routes, a longer and a shorter one. When the subjects were asked to return to the original room, all but one took the same route along which they'd been led -- even when the alternative route was much shorter, and even if the experimenter pointed out that a shorter alternative existed. This tendency to stick to an old route even when shown a faster one also shows up in fish and ants.

In fact, maybe we can take a few lessons from the Formicidae family, a.k.a., the humble ant. A paper appeared on the arXiv last October demonstrating that ants might be better than humans at regulating their own traffic efficiently. Dirk Helbing is a "congestion expert" at Dresden University of Technology (there's a lot of good traffic research going on in Germany), who teamed up with a few colleagues to build a tiny ant motorway in the lab, featuring several "carriageways" between an ant nest and a source of sugar.  It didn't take long for a few ants to find the shortest route to the sugar, leaving a handy pheremone trail for their friends to follow. And eventually so many ants were following that trail it became pretty much saturated with ants.

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

And as result, traffic jams never formed. Somehow the humble ant has cracked on of the most challenging problems in traffic physics, not to mention the routing of data over the Internet and other networked systems: "the efficient distribution of limited resources by decentralized, individual decisions."

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

So why not build more roads, or widen the highways to accommodate more cars? It turns out this might not be an optimal solution, either, according to a paper in Physical Review Letters last August (2008 was a good year for traffic research). A collaboration of Korean and US physicists conducted a study and concluded that building more roads can actually make traffic congestion worse. It's a paradox, stemming from the fact that individual drivers act solely in their own best interest -- say, the quickest route for them -- and instead end up slowing traffic as a whole. You know, like those jerks who drive along the shoulder or an exit lane during heavy traffic to sneak a few spots ahead in line, thereby slowing the methodical merging to nearly a standstill -- because it's all about their needs, y'all. If someone cut in line in the grocery store, they would be shamed into at least feeling a twinge of guilt about it, and I've seen cashiers actually toss those customers to the back of the line in retaliation for their rudeness. But in a car, you have the protection of privacy -- mobile privacy -- and the offender doesn't stick around long enough for the social peer pressure to kick in.

It's basically a conflict of interest between individual and collective benefits -- the researchers dubbed it "the Price of Anarchy" -- and the result is 30% longer commute times overall. How do we change human nature, which seems to act counter to the collective best interest when it comes to traffic? Well, you can try to force a change in behavior, like the ants. The Korean/US team found that by simply shutting down a few select streets, they eliminated certain travel options, thereby bringing the interests of single drivers more in line with the interests of collective commuters as a whole.

Here's their illustration. Select starting and ending points linked by two different routes: one over a short but narrow bridge, and a longer one over a wide freeway. If half of the drivers choose the bridge and the other half choose the highway, the combined travel for all is minimized -- except many more drivers opt for the shorter route over the narrow bridge, which quickly becomes clogged. When it gets clogged enough, some of the drivers (that could be me!) switch back to the highway. This process plays itself out over time until the traffic reaches some sort of equilibrium, a state in which no one driver can reduce their commute simply by switching to the other one. But the combined total commute time is significantly longer even after all that switching than if the drivers had just split down the middle over each route to begin with. Shutting down carefully selected streets can encourage drivers to make the optimal choices -- without realizing they are doing so.

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

Basically, ants are better at regulating their behavior for the common good. They even will use their tiny bodies to plug potholes in the trail leading back to the nest, for faster, more efficient delivery of food. Human beings, in contrast, constantly struggle with the conflict between the common good and "What's in it for me?" We could learn a few things from the ants.

road rage redux

It's fun to snicker at bad (or just mediocre) movies, especially those that take themselves a bit too seriously. But that doesn't mean those movies can't spark interesting discussions on important issues on occasion. Take the 1997 Demi Moore vehicle, G.I. Jane, as a case in point. Sure, the premise is a bit far-fetched: Demi Moore plays a female soldier determined to be become a Navy Seal by enduring a grueling all-male boot camp. (Jen-Luc Piquant doesn't find this the least bit far-fetched, but then, she's a bit of a scrapper herself, and still bitter that her plans to found an uber-tough Cybergang called the Virtual Visigoths hasn't met with more enthusiasm.) The writing is pedestrian, the tone often a bit too melodramatic. We are not offering it up as an example of High Cinematic Art. But there's nonetheless something viscerally appealing about it, and in between the overwrought cliches, there's some very nice moments, such as Demi shaving her head as a symbol of renouncing her femininity and becoming All Soldier (an entire blog post in itself). Then there's Demi getting the crap beaten out of her by the drill sergeant, going to a bar afterwards and running into another woman in the ladies room. The woman takes one look at her battered face and says, "Honey, it's none of my business, but if I were you I'd leave the bastard."

My favorite scene is when a female military counselor asks Demi's character (Jane) why she's doing this to herself. "Do you ask the men the same question?" Jane says. The counselor says she does, and notes that the guys invariably answer, "'Cause I get to blow shit up." Jane shrugs: "Well, there you go."  I like that scene because, while the stereotype is that men like to watch big explosions, the truth is, this is almost a universal human fascination. I'd argue that, under properly controlled conditions (i.e., onscreen in a movie theater), it can be strangely cathartic. (Not that we're engaging in this kind of behavior, mind you; we limit our explosive experiments to Mentos and Diet Coke.)

Perhaps that's why project officials in the DC area have just announced a contest, in which one lucky frustrated commuter will win the opportunity to detonate part of the old Woodrow Wilson Bridge that connects downtown DC with the suburb of Alexandria, Virginia. I kid you not: drivers are invited to fill out forms detailing their horrific commuter experiences using the old bridge, and the winner will get to trigger an explosive charge that will reduce a half-mile section of the structure to rubble. It's the ultimate act of road rage, and it's fully sanctioned by the bureaucratic Powers That Be in a rare show of solidarity with the long-suffering public. Per project spokesman John Undeland: "This is a thank-you and a recognition for all the folks who have had to endure a miserable time getting across the old bridge." (Say it with me: "Hoo-yah, Master Chief!") It all goes down at midnight on August 24th, and Jen-Luc Piquant and I will most certainly be on hand in the "public viewing area" on Washington Street to pay our final respects.

For non-locals, the old Wilson Bridge (constructed in 1961) has been a notorious commuter nightmare, prone to frequent traffic jams and long delays. The Washington Post article linked above features the experiences of Virginia resident Jack Hay, who has commuted back and forth from Maryland and Virginia every day since 1982. Hay estimates that he spent over an hour each day stuck in traffic near the bridge; over 24 years, that adds up to losing about a year of his life to traffic jams along the Wilson Bridge. And that's a whole year of his life he'll never get back. Ponder that next time you're in a jam, and watch your blood pressure rise to unprecedented levels. The new Wilson Bridge is a $2.4 billion project to alleviate all that backed up traffic. The first six-lane span of the new bridge opened earlier this summer, but the second span won't be done until 2008, so motorists have a bit longer to wait before true relief is at hand.

In general, traffic in Virginia has gotten so bad that it became a major issue in the most recent gubernatorial election. Nor is this merely a local problem: it's happening in most major urban centers, in part because new highway construction in the US just hasn't kept pace with increases in population and the number of cars on existing highways. In fact, from 1980 to 1999, the total number of miles of vehicle travel increased by 76%, while the total length of highway miles increased a measly 1.5%. The Washington, DC, region currently ranks third among the top 5 cities with the worst traffic flows. Los Angeles, not surprisingly, tops that list, with San Francisco placing second, and Atlanta and Chicago falling into fourth and fifth place, respectively. (We're frankly surprised New York City didn't make the Top Five, or our former hometown of Seattle: these days, the traffic on I-5 is congested even when it's not prime commuter time, and some local motorists are convinced that the notorious "S" curves along I-405 were designed by the spawn of Satan.)

Apart from quality of life issues, why should we care? Well, it's expensive! The Texas Transportation Institute estimated that in 2000 alone, the 75 largest metropolitan areas experienced a collective 3.6 billion hours of delay. That translates into 5.7 billion gallons of wasted fuel and more than $78 billion. The average US motorist spends 36 hours every year in traffic delays. In fact, it costs an individual driver living in a large city an estimated $1000 every year, especially when you factor in wasted productivity.

So traffic is important, people. Fortunately, physicists are hot on the case. Traffic physics has been a fairly hot area of research for almost the last ten years. Conventional scientific wisdom has compared traffic jams to the process of freezing -- namely, a phase transition between a liquid to a solid.

Think of it this way: On a sparsely populated highway the cars are far apart and can move at whatever speed they choose while freely maneuvering between lanes -- much like the movement of molecules in a gas. In heavier traffic, the "car molecules" are more densely packed, with less room to maneuver, so cars move at slower average speeds and traffic behaves more like a liquid. If the the "car molecules" become too densely packed, their speed is reduced, and their range of movement is restricted, to such an extent that they can "crystallize" into a solid. So traffic jams aren't random. There's a threshold "value" to the flux of cars traveling along a highway, and if that threshold is exceeded sufficiently -- if local perturbations are large enough -- then the flowing "liquid" traffic jams into a "solid," akin to the critical temperature/pressure point threshold where water turns into ice.

It's a useful but rather crude analogy. The situation is a bit more complicated than that, and scientists would love to understand the phenomenon in far greater detail. A major breakthrough occurred in 1998, when a physicist named Boris Kerner with the Daimler Benz Research Institute in Stuttgart, Germany, published a paper in Physical Review Letters. (Kerner has since written an entire book on the subject, called The Physics of Traffic: Empirical Freeway Pattern Features, Engineering Applications and Theory.) Kerner analyzed data collected from several years of traffic monitored along German highways and found that traffic tends to follow the physics of self organization.

Based on that data, he developed a model that essentially broke traffic into three basic categories: freely flowing, jammed (solid state), and a bizarre intermediate state called synchronized flow, in which densely packed "car molecules" move in unison, like members of a marching band (or the highly disciplined troops in G.I. Jane). When this happens -- when all the cars are traveling at close to the same average speed because of the vehicle density on the roadway -- they become highly dependent on one another. A physicist might compare the relationship to the correlated motion of electrons in metals, which gives rise to weird phenomena like superconductivity.

Highly correlated traffic means that a tiny perturbation -- a butterfly flapping its wings, or a single driver braking unexpectedly to change a CD -- will send little ripples of corresponding slowdowns through the entire chain of cars behind him/her. That's one reason why slowdowns and traffic jams occur most commonly at merge points, especially exit and entrance ramps, or when lanes are closed due to road (or bridge!) construction. A state of steady synchronized flow, punctuated by these tiny ripple effects ("narrow jams") can persist indefinitely, but the balance is delicate and highly unstable. If the volume of cars continues to increase, the density continues to increase, and eventually you get a "pinch effect" -- that frustrating "stop and go" phenomenon, in which you escape one narrow jam only to encounter another a little further down toe road, until they all converge into a single wide jam. Traffic comes to a standstill. Collective road rage may ensue.

As Kerner and his fellow traffic-obsessed physicists can attest, a key component to building better traffic models is collecting better real-world data. That's another expensive undertaking: State and federal agencies spend about $750 million each year on traffic monitoring. So they're always on the lookout for new, innovative yet inexpensive solutions to the data collection problem. Things like cellular phones, which have become pretty much ubiquitous in modern society.

Several state transportation agencies (Maryland, Virginia, Missouri, and Georgia) are experimenting with new software that uses radio signals from drivers' cell phones as tracking devices, so they can better monitor traffic patterns. Officials are quick to point out that they are not monitoring actual conversations, just the radio signals transmitted by the phones; the phones don't even need to be in use, just turned on. "Listening posts" are placed throughout a region capable of detecting but not sending radio signals. It will pick up a cell phone signal and time-stamp the signal's arrival. By analyzing how long it takes the radio wave to reach the listening post from the cell phone, a computer can calculate almost precisely where that phone is located on the highway. You need three such posts to determine a 2D position of a cell phone user. Adding radio tags along the highways to time when vehicles pass between given points can determine the car's location and speed. All this information can be collected and disseminated via Web sites, electronic road signs, even sent to cell phone users who sign up for customized traffic reports.

It all sounds just a wee bit "Big Brother"-ish to us. And in Berkeley, California, they're taking the wireless option even further with a testbed project dubbed "Smart Cars and Smart Roads, part of the Vehicle Infrastructure Integration Initiative. Participating cars would be equipped with technology to pick up signals transmitted form the road on which they are traveling, thereby relaying critical information, like whether there's an accident or icy road (in California?) up ahead. In this way, the cars themselves would serve as anonymous data collectors.

Kerner's self-organizing model would seem to indicate that while drivers think they are acting of their own accord, they are really just behaving like a tiny unthinking particle, a single grain of sand in an enormous sand pile, unconsciously following the "rules" of self-organized flow. But that's not quite the case: driver behavior really can impact traffic patterns in significant ways. The unpredictability of human behavior is one reason why many traffic models aren't as accurate in their predictions as they could be. In 2004, a team of German scientists led by Michael Schreckenberg (University of Duisberg-Essen) sought to rectify that.

They came up with new predictive models that took realistic driver behavior into account: you know, all those jerks on the highway who keep changing lanes and cutting in front of you just when your lane is starting to move, which in turn arouses your competitive instincts so that you (a) honk angrily and tailgate him for a few yards, determined not to let any other "cheaters" infringe on your valuable space, or (b) switch over to a new lane in turn, thereby causing another slowdown. Not that we're bitter or anything. As team member Robert Barlovic put it, "Real drivers tend to hinder each other when doing things like changing lanes."

The model was quickly put into use to forecast traffic along the autobahn network around Cologne, based on real-time traffic data gathered by  embedded sensors in the road. Our German readers (both of you!) can check these forecasts -- up to an hour ahead -- on this handy Website. More than 300,000 people are already doing so. The problem is that this broad access to more accurate information is actually changing traffic patterns. People modify their behavior based on the new information provided by the forecast models: they all flock to the same exits to avoid upcoming congestion, for example. This in turn eventually makes those models less accurate in their predictions. More information is not, as it turns out, the answer to all our traffic woes.

The good news is we are not helpless grains of sand in an endless flow of avalanche dynamics. A single driver resisting the competitive urge really can make a difference. William Beaty, the self-proclaimed "Science Hobbyist," maintains a fascinating Web page ("Physics for Bored Commuters") with his own informal "experiments" on the physics of traffic flow, illustrated with cute little animations. (Jen-Luc Piquant obviously doesn't drive on anything other than the Information Superhighway, but even she has been able to experiment via this nifty online traffic game.) Beaty offers these handy tips: (1) Maintain a large space ahead of your car. (2) Encourage one, two, or even three cars to merge ahead of you. Yes, we know this is hard, but try to be the bigger person. (3) If traffic slows to a complete stop, still try to keep two car-lengths of space open ahead of you. (4) Never "punish" merging drivers by closing your gap. Apparently, this really works. People cause the problems, but people can also be part of the solution.

I find that strangely comforting today, in the wake of disturbing headlines about a thwarted planned airline terrorist plot in the U.K. Needless to say, airport security lines are experiencing their own "solid state" traffic, as panicked officials confiscate such mundane items as shaving cream, shampoo, contact lens solution, hairspray, and bottles of soda or water -- in short, any liquids or gels. Look, I realize that these threats are serious, and human lives are at stake, but this is getting out of hand. I'm all for taking stringent measures to protect travelers from the threat of terrorist attack, but I doubt very much that taking away my shampoo will make the skies one whit safer, any more than collecting tons more data for predictive models will automatically solve our traffic woes.

Panic and fear over the potential for terrorist attacks has been encroaching, inch by inch, on our personal freedoms, not to mention those tiny day-to-day conveniences one takes for granted until they're gone. The result is a drastically reduced quality of life. A friend of mine once observed that the only truly safe and secure society would be a fascist society. He wasn't advocating that, mind you -- just making the observation that security comes at a cost to personal freedom and convenience. There's always a tradeoff that must be made. But what will we be asked to give up next in the name of our own "safety"? Why not just go all the way and simply ban "People" from boarding aircraft? It is people, after all, not the things they carry, that pose the deadly threats. Anything can be a weapon in adequately trained hands, even a ballpoint pen.

The key word here is "panic."  Terrorists traffic in the psychology of fear, and if we allow ourselves to give into fear -- well, then the terrorists win, even if they don't succeed in their nefarious plotting. The complexity of the physics of traffic flow demonstrates that we are not helpless grains of sand, doomed to unconsciously follow preset behavior patterns. Let's choose to be part of the solution, not the problem, and keep our collective cool.