I’ve always been fascinated by traffic. (I grew up in LA, so I had plenty of time to indulge my interest.) City streets are a complex system in which seemingly insignificant changes – a broken street light, a stalled car, a poorly designed highway merge – can have dramatic consequences. In this sense, it’s a useful metaphor for all sorts of intricate systems, from gene regulation to neural networks.
Perhaps the single greatest mystery of my childhood was this: Why do freeways get clogged when there isn’t an accident? I would fantasize on my way to elementary school – we would often be sitting in gridlock on the 101 – of getting a loudspeaker and ordering every car to accelerate. I figured if we all started moving at the same time and at the same speed then the traffic would disappear.
My latest Seed column – on newsstands now – explores some new research on the origins of traffic jams.
A few years ago, the Swiss economists Bruno Frey and Alois Stutzer announced the discovery of a new human foible, which they called “the commuters paradox”. They found that, when people are choosing where to live, they consistently underestimate the pain of a long commute. This leads people to mistakenly believe that the McMansion in the suburbs, with its extra bedroom and sprawling lawn, will make them happier, even though it might force them to drive an additional forty-five minutes to work. It turns out, however, that traffic is torture, and the big house isn’t worth it. According to the calculations of Frey and Stutzer, a person with a one-hour commute has to earn 40 percent more money to be as satisfied with life as someone who walks to the office.
The reason long commutes make us so unhappy is that the flow of traffic is inherently unpredictable. As a result, we never adapt to the suffering of rush hour. (Ironically, if traffic was always bad, and not just usually bad, it would be easier to deal with.) As the Harvard psychologist Daniel Gilbert notes, “Driving in traffic is a different kind of hell every day.”
But why is traffic so unpredictable? After all, the number of cars on a highway during a typical weekday rush hour is fairly constant. And yet, even when there are no accidents – and most traffic isn’t caused by collisions – the speed of traffic can undergo dramatic and seemingly inexplicable shifts.
The key to understanding traffic jams is something known as “critical density,” or the number of vehicles that any road can efficiently accommodate. When this threshold is crossed – when too many cars are trying to cram onto the same six lanes of asphalt – the flow of traffic starts to breakdown. At this point, congestion becomes all but inevitable, as even seemingly insignificant events, such as a single driver tapping on the brakes, can trigger a cascade of brake lights. That’s when the highway becomes a parking lot.
While the concept of critical density has been repeatedly demonstrated using computer simulations – drivers are surprisingly easy to model as a system of interacting particles – it wasn’t until last year that this theory of traffic was experimentally confirmed. A team of physicists at Nagoya University wanted to see how many cars could maintain a constant speed of 19 mph around a short, circular track. It turned out that the critical number was 22: once that density was reached, tiny fluctuations started to reverberate around the track, which caused the occasional spontaneous standstill. As the scientists note, this is actually a pretty familiar scenario for particle physicists, who are used to studying phase transitions, such as the transformation of liquid water into solid ice. In this case, the critical threshold is temperature, which triggers clusters of molecules to slow down and form a crystal lattice, which then spreads to nearby molecules. A traffic jam is simply a solid made of idling cars.
I then explore what we can learn about traffic from ants. And if you haven’t read it, I highly recommend Traffic, by Tom Vanderbilt. Finally, here’s a cool video taking you behind the scenes at LA Traffic control.