Before Yellow: Early Signals Were Basically “Stop” or “Go”

Early traffic control relied heavily on officers directing movement by hand—or on simple two-color signals that flipped between red and green. The problem was scale: as vehicle speeds increased and intersections grew busier, an abrupt change from “go” to “stop” didn’t give drivers enough time to react safely. That gap is exactly where the yellow light comes in.

William Potts and the Birth of the Yellow “Caution” Phase

Histories of U.S. traffic signals commonly credit William Potts—a Detroit police officer and traffic-signal superintendent—with inserting a yellow (amber) caution light between green and red to warn drivers of an impending stop. According to the Library of Congress, Potts introduced the yellow light concept around 1917 and later developed a major advancement: a four-direction, three-color electric traffic signal by 1920.

Transportation museums and historical collections support the reasoning behind the design. Early illuminated signals used only red and green; adding an amber “caution” phase gave drivers time to slow safely and helped the three-color system become the U.S. standard by the mid-1930s.

One widely cited early tri-color, four-direction signal is associated with Detroit. An artifact record from the Henry Ford Museum documents a Potts-era signal design linked to early installations at major Detroit intersections, including Woodward Avenue & Fort Street.

What About Garrett Morgan? (An Important Part of the Story)

You may also see Garrett A. Morgan credited in traffic-signal history—and for good reason. In 1923, Morgan patented a three-position traffic-control device designed to improve intersection safety by introducing a controlled transition phase, including an all-direction stop. According to the Federal Highway Administration, Morgan’s invention influenced later traffic-control systems, even though it differed mechanically from the electric red–yellow–green signals that became standard.

In short: Potts is widely credited with introducing the yellow light into the modern electric three-color sequence, while Morgan is recognized for advancing safer intersection control through a patented three-position signal concept.

Why the Yellow Light Matters for Safety

The yellow phase functions as a built-in reaction buffer. It helps reduce:

• Rear-end collisions caused by surprise stops
• Intersection conflicts from drivers guessing when a signal will change
• Pedestrian risk by clearly signaling when right-of-way is ending

That safety benefit is why the amber light quickly became a standard part of U.S. traffic signals as cities expanded electric traffic control nationwide.

Quick Timeline: How We Got the Modern Signal

• 1917: William Potts introduces the yellow caution-light concept.
• 1920: Potts develops a tri-color, four-direction electric traffic signal in Detroit.
• 1923: Garrett Morgan patents a three-position traffic signal device.

Traffic Safety Takeaway: “Caution” Only Works When Drivers Can See It

The yellow light’s purpose is clarity. In modern work zones and temporary traffic control, that same principle applies: drivers need high-visibility guidance well before they reach the decision point.

Sources & Further Reading (External)

• Henry Ford Museum — Artifact record of a tri-color, four-direction traffic signal attributed to William Potts (view source)
• Library of Congress — “Taking on Traffic: A Closer Look at the Signals” (view source)
• Federal Highway Administration — Garrett Morgan’s traffic-signal legacy (view source)
• History.com — Garrett Morgan patents a three-position traffic signal (view source)