Giz Explains: Traffic

Americans talk about traffic like the English talk about the weather. It's a constant upset, a never-ending obsession. We've heard the stats: The commuter loses 36 hours a year stuck on the road, over $800 dollars in the same amount of time, and, well, an immeasurable amount of sanity. There's a lot of talk about what has to happen to make it better, but not a lot of discussion about what causes the problem in the first place. So why is there so much damn traffic?

Part of it is overcrowded roads, for sure. Rush hour (or, rather, rush hours—there are six of them in a given day) happens because of the whole nine-to-five business. When so many people are expected to come and go at the same time, it creates a high tide of vehicles that flood our roads. But even though crammed motorways account for the bulk of our frustration, they don't explain random Sunday afternoon slowdowns or repeated red lights.

Surface street jams, if not caused by an event, a construction issue, or an accident, can often be blamed on technology. When you feel like you just can't stop hitting. every. red. light, the machines presiding over the signal are likely miscalibrated.

Traffic lights typically work in one of two ways. "Fixed-time" signals are set on a daily red light green light schedule, and "traffic responsive" signals make decisions based on the flow of cars in real time. Both systems screw up.

Giz Explains: Traffic

Fixed time signals work the same way as those light timers you use to confound the criminalz when you leave town: Electrically driven dials tell the lights to turn on and off at specific times on specific days. The dials can be programmed to accommodate the morning rush at a nearby school, the evening commute, or a new shopping center. The problem is that these kinds of timers can easily fall out of sync with the actual time of day at the whim of weather, electrical infrastructure problems, or just plain old mechanical imperfection. So while the dial may think it's regulating traffic at one time, real drivers are encountering awkward slowdowns at another. "There could be very different patterns at 5pm than at 4pm," says Texas Transportation Institute Research Engineer Timothy Lomax. If a dial is off by an hour: boom, traffic.

So the "traffic responsive" signals should be better, right? Well, they come with their own set of difficulties, since they rely on real-time monitoring systems to usher cars through intersections—usually wires in the pavement that count cars teamed up with a video camera that monitors a wider area. The camera always needs to be pointed in the right direction, but unfortunately, high winds can spin the cameras around, aiming them at the corner gas station or the birds overhead. When this happens, you, the driver, are no longer part of the equation. Oh—and you're going to miss your appointment.

The second biggest traffic problem is lane closure. These are your fault—or, more specifically, your crashes, stalls, and breakdowns. They shut off valuable space in an already overcrowded system, making it tough for everyone to move forward. A study out of Marin County showed that blocking a freeway lane for just 15 minutes during the start of peak traffic causes more than three miles of backup, which then takes three hours to clear. [PDF]

The more frustrating problem, though, is that a large chunk of crash-related traffic is needless. When just a few drivers slow because, Holy shit! There's a crash on the other side of the wide median that absolutely doesn't affect me but that I can't stop watching! it can create a lasting and significant freeway-wide reduction in speed.

To understand why this happens, a little history: Seatbelts, airbags, and crash-absorbent frames have made drivers more at ease with their safety on the road, which means that on the freeway, we're driving closer to each other. Today we cram 2300 cars in each lane per hour, up from 1800 back in the day when crashes meant certain death.

Today, we can see taillights several cars ahead through rear and front windshields, so we tailgate. But there's a catch. When one car slows down to gawk at the overturned car across the way, it very quickly sends a wave of brake lights backwards and sideways. This also happens, to a certain extent, when someone brakes for no reason, but in the case of an accident, it causes a chain reaction that propagates the slowing and the rubbernecking. By the time the car at the end of the jam starts moving again, the crash site's been cleared. "For that amount of traffic delay," says Lomax, "you could probably make an economic argument that median vegetation to block the other side would pay for itself."

How about hovercars? How long would they take to pay for themselves?

Original artwork by Christopher Hartelius. For more of Chris's work and other true news stories, please check out his website True American Dog.