There are two kinds of people in this world. One sees a yellow light and slows down. The other sees a yellow light and speeds up.
I'm the first kind, but the second kind is always right behind me. Life's funny like that, I guess.
Now we're both sitting at the red, taking a break. So while we wait, let's answer a few questions you never knew you had about the light in front of you. Enjoy.
Who invented the traffic light?
The first one went up in London on December 10, 1868, outside the Houses of Parliament. Railway engineer John Peake Knight borrowed straight from train signaling. He used semaphore arms by day and a gas lamp showing red or green at night, all worked by hand by a police constable.
Within weeks, gas leaked and the lamp exploded, injuring the officer running it. (No word on his workers' comp claim.)
The light we know today came together over the next 70 years:
Year | What happened |
|---|---|
1868 | London's gas-lit signal debuts, then explodes. |
1914 | The first electric traffic signal goes live in Cleveland on August 5, with red and green lights and a buzzer to warn drivers of a change. |
1920 | Detroit police officer William Potts adds a third, amber light to warn drivers that red is coming. |
1923 | Inventor Garrett Morgan patents a T-shaped signal with an all-stop position and later sells the rights to General Electric for a reported $40,000. |
1935 | The first national Manual on Uniform Traffic Control Devices (MUTCD) makes red, yellow, and green the standard on U.S. roads. The Federal Highway Administration still maintains it today. |
Why is red for stop and green for go?
Blame the railroads. Early train signals used red for stop, since red already meant danger. White meant go, and green meant caution.
White didn't last. If a red lens cracked or fell out, the lamp behind it shone white, telling an engineer to go when he should stop. Railroads moved green to "go" and gave "caution" to yellow. When engineers built signals for roads, they adopted the colors drivers already knew from the tracks.
You may have heard red was chosen because it carries farthest through fog. Red does stand out, but the colors were settled on the railroads long before anyone hung a light over a street.
Position matters as much as color.
About 8% of men and 0.5% of women of Northern European descent have some red-green color deficiency. That's a big reason red sits on top and green on the bottom of every vertical signal. Drivers can read the light by its position even when the colors look alike.
How does a traffic light know you're there?
A lot of them don't. More than half of traffic lights nationwide don't account for congestion or lighter-than-normal traffic. They cycle on fixed timers, which is why you can sit at a red at 2 a.m. with no one else on the road.
The lights that do respond usually use one of these:
Induction loops: Wire coils cut into the pavement near the stop line that sense the metal in a car. Motorcycles and bikes often don't register, which is why riders sometimes wait out a full cycle.
Cameras and radar: Units mounted on the signal arm that count vehicles and measure how long the line is.
Emergency preemption: Transmitters on fire trucks and ambulances that turn lights in their favor.
What's next for the traffic light?
The next big upgrade may not need new hardware at all.
Google is retiming lights with Maps data.
Google's Project Green Light uses Google Maps driving data to spot fixes, like shorter reds during off-peak hours or syncing intersections that aren't coordinated, then hands those recommendations to city engineers. Google's early data showed AI-optimized lights could cut stops by 30% and emissions by 10%.
Michigan is testing lights that read connected cars.
The University of Michigan received a U.S. DOT grant to test signal changes using real-time speed and location data from cars, with a pilot in the Detroit suburb of Birmingham.
Will traffic lights get a fourth, white light?
Researchers at North Carolina State University think they might. In their "white phase" concept, self-driving vehicles coordinate their own movement through an intersection, and the white light tells human drivers to simply follow the car ahead. The team first introduced the idea in 2020, and in their simulations, even a 10% share of autonomous vehicles reduced delays by 3%.
Not everyone is sold. Waymo has cautioned that AV-specific infrastructure investments may turn out to be premature or even unnecessary. For now, red, yellow, and green aren't going anywhere.
Fun facts for your next red light
Syracuse, New York, has an upside-down traffic light on Tipperary Hill, with green above red. Irish residents of the neighborhood kept smashing the red-over-green version, and the city eventually gave in.
A 2022 Inrix analysis found U.S. drivers emit nearly 28 million metric tons of carbon dioxide at red lights every year.
Garrett Morgan also invented a safety hood that he personally wore to rescue workers trapped in a Cleveland tunnel explosion in 1916.
Roses are red,
Violets are blue,
Traffic lights are red, yellow, and green,
Lots of things are colorful.
Light's green. Go.
