The Hidden Genius and Influence of the Traffic Light


See it sway: three-eyed blind bat hanging from a wire. Or perhaps there: perched atop a pole, lights moving from top to bottom–green yellow red green yellow red–in its unvarying sequence. Two hundred years ago, it would have been a wonder, something on display at the Great Exhibition of 1851, gawked at by Victorians. Today it’s seen but unconsidered, passed under a dozen times a day by most of us, influencing how we move, shaping our cities, warping how we travel, and occasionally, inadvertently, helping to kill us. Consider the traffic light.

Is there anything lonelier than a solitary traffic light blinking to an empty road? It’s an establishing shot that screams: desolation. It plays on our fear that the mechanical world doesn’t care about us, and will exist long after we’re gone. The traffic light doesn’t need people. And yet.

Here’s the thing: traffic lights don’t really do anything.

They don’t–can’t–physically stop you. They don’t engage a barrier to prevent cars from going through the intersection, and lower that barrier when it’s time to drive. They won’t stop you from plowing into a hapless pedestrian at the intersection. No, they’re only a mechanical prop, a signifier of a social contract we’ve agreed to (and have written into law). They are a means of behavior change, and we (mostly) obey.

It is hard to think of a technology so widely adopted, so ubiquitous, so influential, and, yes, so well designed. Traffic lights are used by billions daily, even by the illiterate and unschooled. The lights are designed to be seen even in broad daylight, sometimes by using cap visors or, more recently, bright LEDs to make them visible. They can be aimed on many lanes of traffic or just one, with special Fresnel lenses like those used in lighthouses to focus the light on an intended viewing area and obscure it from other lanes of traffic.The traffic light has evolved from dumb electro-mechanical objects to smarter, networked ones, able to adapt to the environment in sophisticated ways.

The Genius of the Yellow Light

Originally, there were no traffic lights. For almost forty years we drove cars without them—not to mention the several millennia we rode horses sans lights, or any intersection control at all. When the street light was invented by Lester Wire in Salt Lake City in 1912, they had no yellow lights, only green and red, with a buzzer to let drivers know the light was about to change. It didn’t take long to realize that this probably wasn’t the best solution, and thus the yellow light was born.

Red, at least in the West, has been the color of danger since at least the Romans. Green as “go” came from railroad signals, color-blind people be damned. But yellow is the easiest color to see. It’s the first color your eye can detect, because physics: more light is reflected from bright colors and thus with yellow, eyes become more stimulated.

The yellow light is by far the most sophisticated and cognitively challenging part of any traffic light. Red and green lights have had to consider timing, namely: how long should one side of the intersection remain green, the other red. This creates the “capacity” of a signal: how many vehicles can move through on a single change of the light. This, in turn, creates (or disrupts) flow throughout the entire traffic grid of a city. Longer green lights mean more vehicles move through the intersection. If one light is letting too many cars through, the next light might have traffic jams as cars pile up. This is how traffic can be (partially) controlled: by adjusting the capacity of traffic lights, letting more or less traffic pass through.

Sometimes it really would be more dangerous to stop than to run a yellow light.

The yellow light doesn’t really control capacity, but instead creates an ephemeral Zone of Decision around the intersection. When a light turns yellow, nearby drivers have a choice to make, quickly: do I speed up and drive through the yellow light, or do I slow down and stop? Driving instructors will of course always tell you that a yellow light means slow down and prepare to stop, but on the street, that’s not always how it works. Sometimes it really would be more dangerous to stop than to run the yellow. And sometimes those driving instructors are right: running the yellow is a terrible, dangerous idea. How do you know which is which?

Yellow lights generally last three to five seconds. Which means, when one appears, in the space of about a second, you’ve got to do a few intense calculations: how far you are away from the intersection, how fast you’re going, how clear the intersection is, and, increasingly, is there a camera that will take a picture of me running a red light if I time this wrong? This moment is the Zone of Decision. Guess wrong, and cars crash and people can get hurt or die, as thousands do every year.

In the near future, it might not be you making those decisions, but the vehicle itself. Audi has introduced a technology that lets cars connect to traffic lights, displaying a countdown on the dashboard to let drivers know when the light will turn green. It’s not much of a stretch to having it know if you can make the yellow light or not. Many things are possible once traffic lights and vehicles can communicate to each other.

The Internet of Traffic Lights

Our relationship to traffic lights is changing. While many urban traffic lights have been computerized for decades (and thus you can have improvements like pedestrian crossing buttons and different light durations at different times of the day), the interplay between vehicles, pedestrians, and traffic lights is becoming more sophisticated. Cameras and other sensors built into the street or into the traffic lights themselves can detect the presence of cars, bikes, and pedestrians, and adjust accordingly. These coordinated signal systems try to ensure that lights don’t turn red just as cars are arriving, or have a car waiting at a red light when no cross-traffic is going through the intersection. This will save gas, time, and simply driver annoyance.

Traffic lights are only a mechanical prop, a signifier of a social contract we’ve agreed to (and have written into law).

Of course, once a majority of vehicles (including bicycles) are connected to the internet, they could signal their arrival time and route to nearby traffic signals, allowing them to adjust accordingly. Once the traffic grid understands where people are going, it could very well make getting to destinations much faster and more eco-friendly. The US Department of Transportation estimates that responsive traffic signals could save as much as 10 percent of all motor fuel consumed–17 billion gallons a year.

The rollout of smart traffic lights has been slow, with some complaints that the lights are too unpredictable. Like the buzzer that was the original yellow light, there’s likely to be some missteps in the adoption of these new traffic lights. They might require a redesign, adding new signals and cues to let us know what they’re up to. And while some cities just might take Hans Mondeman up on his suggestion to remove all traffic lights—a suggestion that makes more sense with self-driving vehicles—the lights over our streets are likely here to stay (for now).

Will We Eventually Move Past the Traffic Light?

Some have argued that we should do away with traffic lights, insisting they actually make us less safe and hinder traffic flow, that we’ve placed personal responsibility for speed and caution into these devices, and that without them we’d bother to pay attention to the environment outside the car.

The patron saint of this philosophy was the late Dutchman Hans Monderman, who believed that our world was divided into two parts: a Traffic Space that was designed for automobiles and a Shared Space for people and automobiles. Traffic Space was the realm of highways and overpasses, while cities and towns were Shared Space. One was built for vehicles, another for people. Cars are guests in Shared Space, and the urban architecture should support that. Traffic lights are firmly in Traffic Space, and thus, the argument goes, have no place in cities. (Problematically, Monderman doesn’t seem concerned with low-density areas like the countryside where people also live and drive.)

In some towns where removing the traffic lights has been tried, the results seem to bear Monderman out. They are safer, and traffic still seems to flow. For example, at one intersection where traffic lights were removed, accidents dropped from nine a year to one. But it seems unlikely–at least until cities only allow self-driving cars into them–that most traffic lights will go away.

The Hidden Genius and Influence of the Traffic Light was originally published on WIRED.

Image: Ingo Jezierski/Getty via WIRED