An Amber-Colored Conundrum

Jul 05, 2004

RAY: A few weeks ago, my younger son Andrew, who completed his first junior year in college, decided to come home for a few days to reacquaint himself with his parents.

TOM: Who, of course, are paying his tuition!

RAY: Right. He wanted to refresh his memory in case he had to identify us at the inquest.

He and I were driving around town, and he said, "Gee, I hadn't noticed that our town has changed a lot of its traffic signals to the new lights."

Now, traffic lights basically consist of a lens, like a green or a yellow, or a red lens, and a light bulb behind the lens. When the bulb goes on, you see the green, or the yellow, or the red light. The new traffic lights, however, are light-emitting diodes. Instead of having one bulb, you have perhaps dozens of diodes that are shining the light. The new lights are much cheaper to operate and they're much brighter.

TOM: And they don't all blow out at once!

RAY: Exactly. But, we noticed that there are some of the old lights still around. In fact, where they have put new ones in, they've replaced the green light and they've replaced the red light, but they haven't replaced the yellow lights.

I say, "It must be that they don't make light emitting diodes in yellow...maybe that's why they haven't replaced them."

We drove along a little farther, and Andrew says...

TOM: "Au contraire, piston puss!"

RAY: He says, "I think I have a better answer than that."

And, I said, "Yeah, it's about time." And he said, "As a matter of fact, it is about time." And that's the hint.

Why do we see new green lights and new red lights, but no new yellow lights?
RAY: I said, "It's about time." And that was a hint. Why do we see the new LED green lights and red lights, but no new LED yellow lights on? Here's the answer my son came up with, and I liked it.

Most towns are probably replacing these lights on some kind of a schedule. That is, either when the lights have actually burned out or when they're expected to burn out. So, if a typical conventional light is expected to last, say, five years, maybe at the four-year and eleven-month mark they install one of these new LED lights.

But the yellow lights never burn out. They're hardly ever on, think about it. What's on most of the time?

TOM: Red and green.

RAY: Right, and the yellow lights are on for just a few seconds, then they go out. Do we have a winner?

TOM: We do. The winner is Adam Ellsworth from Sioux Falls, South Dakota.

RAY: Where they have one traffic light.

TOM: For having his answer selected at random from all the correct answers that we got, Adam will get a $26-dollar gift certificate to the Shameless Commerce Division of, our esteemed web site. And he can get our new Car Talk middle-age male skin cancer prevention system.

RAY: Is this some kind of medical device?

TOM: No, it's a baseball hat. A nice looking one, too.

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