crosstown traffic

FrazzledJen-Luc2 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. 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.

10 thoughts on “crosstown traffic”

  1. This article reminds me of a website I read about 8 years ago: http://trafficwaves.org/ This redefined the way I drive, since it illustrates quite nicely how one person can potentially eradicate a traffic wave or jam at the expense of a few minutes of commute time. In essence, altruistic driving taken to its most logical conclusion. Also, it doesn’t hurt that it increases fuel efficiency significantly.

  2. I usually resist going to meetings at UCLA because of traffic in I-10 and rarely do so. You might consider surface streets (Olympic or Pico Blvd). I shaved 30 minutes by doing that on one trip.
    An unexpected bonus that morning was getting lost in Beverly Hills while trying to find Sunset Blvd. I didn’t know precisely where I was and didn’t really care. I had that saved time and the neighborhoods were amazing.

  3. There you go again, politicizing da science. 🙂
    And Ken, thanks SO much for spilling the beans on my secret route; Pico is the best by far. But now EVERYONE’s going to be taking Pico. 🙂

  4. A very enjoyable article. These are the answers to the questions I have when I get stuck in traffic (which thankfully is very rare)

  5. Yet another reason Ants are freaking awesome. Dawkins gives a mind-blowing explanation of ants and other hive insects in “The Selfish Gene” and shows how “hive mentality”, which is seemingly counter intuitive to the idea of natural selection, is actually one of the BEST functioning ways of ensuring one’s own genes pass on to the next generation. The perfect nexus of socialism and selfishness. It’s really a good thing these bastards are so small or they would seriously rule the earth.

  6. You should read “Traffic” by Tom Vanderbilt! (http://www.howwedrive.com/ is the blog.) I picked it up at the library a few weeks ago, and it’s one of those books that you babble about at everybody for a while thereafter. Lots and lots of stuff about why building more roads makes things worse, the psychology of merging, the role of corruption in bad traffic, etc., etc.
    (And is there no public transportation option? I took the bus to school in Altadena/Pasadena when I was in high school, when it was still the RTD, and my kid sis took the Gold Line from Pasadena to CSU Northridge.)

  7. Thanks for the book recommendation… And no, there is not really a workable public transportation option that would be preferable to me daily commute. The Gold Line is great, but goes only to very limited areas…

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