Traffic lights

As part of our ongoing obsession with rethinking cities, we’re bringing you a six-week bonus series on the future of mobility. It will cruise into your inbox on Mondays, while the regular edition will arrive on Wednesday as usual.

The streets of New York City in the early 20th century were an unruly cacophony: Before the adoption of traffic signaling systems, horses, carriages, pedestrians, street cars, bicycles, and automobiles vied for right of way. Not only did this often result in collisions, it hindered commutes and hurt local businesses.

All this changed with the introduction of a hulking bronze tower on Fifth Avenue, a gift from the New York commissioner of traffic, which used a simple two-light signal to impose order and alleviate gridlock. The signaling system completely transformed the street scene, increasing the safety, reliability, and efficiency of automobile travel.

In the 1920s, cities around the world started adopting traffic lights. Since then, not too much has changed about traffic signal operation; though they’ve been computerized, modern systems still follow the same general principles as the original 1912 invention.

But for how long? Now that we’re facing a new generation of unprecedented urban growth that relies more heavily on other modes of transportation, the traffic light as we know it may soon become a thing of the past. 

Your light is green. Let’s get moving.

60–90: Ideal signal cycle length in seconds for urban areas, as recommended by the National Association of City Transportation Officials

6,252: Intersections with traffic lights in London as of 2015

12,460: Intersections with traffic lights in New York City as of June 30, 2011

$126,000: Cost of New York City’s second traffic light, installed in 1922

40: Minutes it took to get from 57th to 34th Street on Fifth Avenue before New York City installed its first traffic light

15: Minutes it took to travel the same distance after two traffic lights were installed

8-22: Minutes it took to travel the same stretch during rush hour in 2018

4-12: Minutes it takes to travel the same stretch today

Can you identify this US city from our viz of its traffic signals? How many signals does the city have?

1868: A British railroad engineer modifies a railroad signaling system for use outside London’s House of Parliament. It is operated manually by a traffic cop, and explodes one month after installation.

1912: A policeman in Salt Lake City, Utah uses a wooden box and two light bulbs dipped in red and green paint to create an electrical traffic signaling system. Another policeman in Cleveland, Ohio independently designs an electric traffic light with four pairs of red and green lights, claiming the official title of the world’s first electric traffic signal.

1920: A policeman (once again) in Detroit, Michigan adapts the red-green-amber color scheme of railroad signaling systems to invent the first automatic four-way, three-colored traffic light. The first installation proves a great success, fueling 14 more within the year.

1923: Garrett Morgan, an African-American inventor and newspaper owner, independently converges on the same three-color design after observing too many collisions at the two-signal intersections in Cleveland. General Electric purchases the patent for his invention and begins to build a monopoly on traffic light manufacturing.

1926: The first traffic lights in Britain are deployed in Piccadilly Circus.

1929: New York City dismantles the signaling towers on Fifth Avenue and adopts a simple red-and-green light design.

1950s: New York City adopts the modern three-color traffic signal.

1952: Denver, Colorado introduces a computerized system in which 120 lights are controlled by one computer. Light systems become increasingly computerized over the next two decades, allowing them to monitor and change signals according to real-time traffic conditions.

2020: Google Maps now show stoplights in the US, though far fewer people are on the road due to the Covid-19 pandemic.

There are three components to a traffic light: the controller (the brains), the light head, and a detector. In a fixed time traffic light, as the name suggests, the signal cycles through each light for set durations regardless of traffic demand. This works fine in heavily congested areas but is wasteful at intersections that merge in lightly trafficked roads. If no vehicles are waiting to merge, there’s no need to grant them a green light and hold up vehicles along the main direction of traffic.

As an alternative, a vehicle actuation design takes into account the level of real-time congestion. When a car approaches, the detector sends a signal to the controller to determine how to allocate green time. Consecutive approaches increase the green light phase until the duration hits a predefined maximum or an approach from the other direction sends a conflicting demand.

VA designs are more responsive than fixed time, but they can also be inefficient in negotiating demand and cause a buildup of cars in either direction. They also require regular adjustments to adapt to changing traffic patterns. This labor-intensive task often falls to the bottom of the priorities list, causing VA lights to lose efficiency over time.

In recent decades, the dramatic increase in available computation power has opened new opportunities for more sophisticated management systems. Alibaba, China’s largest online retailer, for example, has a system called City Brain that uses artificial intelligence. Using data from video footage, traffic bureaus, public transportation systems, and mapping apps, City Brain can make live traffic predictions, optimize traffic flow, and detect traffic incidents. After its installation in Hangzhou, China—where the company is headquartered—average traffic speeds increased 15%, and traffic violations were reported with 92% accuracy.

Alibaba has since deployed City Brain in other cities across Asia, including Suzhou, China and Kuala Lumpur, Malaysia, to help alleviate congestion and pollution.

Nothing seems more universal than the color scheme of traffic lights—a symbol understood across countries and cultures. But early traffic lights didn’t have the same uniformity; they used various color schemes to convey their commands.

How we ultimately landed on today’s color scheme is thanks in large part to railroad signaling systems, which used red, green, and yellow—the longest wavelengths of the visible spectrum—out of necessity for signaling across long distances. According to Mental Floss, the custom was first adopted from British steamships, which displayed red lights on their left side and green ones on the right, which signaled it was okay to pass on that side.

The original railroad systems used red for stop, green for caution, and white for go. But white lights proved iffy: Legend has it that green replaced white for go and yellow was switched in for caution after a lost red lens caused a disastrous crash sometime around 1914.

The red-yellow-green color scheme’s global adoption wasn’t without resistance, however. During China’s Cultural Revolution in the 1960s, Chairman Mao Zedong’s Red Guard tried to reverse the red and green signals, so red, the color of the revolution, would signify going forward. Their campaign never came to fruition.

In Japan, some traffic lights use blue to signal “go” because the language’s original word for green encompassed both green and blue in its meaning.

Some experts envision a not-so-distant future system in which traffic lights and cars communicate directly over wireless connection. In Las Vegas, such a pilot is already underway using technology provided by Siemens. The city hopes the experiment will improve road safety in its busy downtown.

Other experts suggest the complete elimination of traffic lights, once all cars are connected and self-driving. Cars would no longer need a centralized conductor if they could communicate and coordinate among themselves instead. 

If autonomous vehicles are coming, cities need to be ready. In 2016, a group of MIT researchers envisioned “slot-based” traffic intersections would double the capacity of traditional ones and thus significantly reduce congestion.

“Remove all the traffic lights, yellow lines, one-way systems and road markings, and let blissful anarchy prevail. I imagine it would produce a kind of harmony.”

— English novelist Sadie Jones on what she would do as mayor for a day

In our last poll, about lungs, 52% of you said you are happy to take a few breaths and appreciate your lungs when you get reminders like ours. Aww!

🏆 The city in Extra Credit is San Francisco and it has 1,397 traffic lights in total. (Not every one is pictured, so please don’t count.) Your prize for playing is this handy party fact: San Francisco was recently ranked the fifth most congested city in the world.