What's the Deal With the Little Blue Light on Stop Lights?

Each morning as I sit at the intersection of Southeast Division Street and Ladd Avenue, dutifully waiting for the signal to tell me that I can safely cross, a little blue light sitting atop the stop light glows. I assume this is not just a light to banish the darkness—it serves a purpose, but what?

—Sarah R.

Don't feel bad, Sarah—that blue light has caused more misunderstandings than my hastily recalled children's book, Horton Hires a Hooker.

What you saw was a "detector confirmation feedback device." Here's the deal:

We all know that some intersections include detectors in the roadway that can tell whether there's a car waiting for a green light. These detectors, called induction loops, use the magic of electromagnetism to tell when there's something metallic overhead.

You may also have noticed that smaller vehicles, like bikes and motorcycles, don't always trip the detector. This leaves the rider hanging, and she eventually runs the red light in frustration.

Most people think that smaller vehicles aren't heavy enough to set off the detector, but the real problem is that they don't cover as much of the roadway. If placed in exactly the right spot, a motorcycle or bike will set off the induction loop.

Why don't the authorities just mark that spot, then? Actually, they do—it's just that they mark it using international-symbol-ese, a language that avoids bias by being equally incomprehensible to all cultures.

The current symbol is a bike and rider with short vertical lines above and below. Put your bike's tires on those lines, and the loop will detect you—obviously! How stupid of you to miss it. (In fairness to the Bureau of Transportation, it's actively working on a more easily understood marker.)

In any case, the blue light confirms that the detector has seen you, and will change the signal shortly. Tell your friends; maybe someday this information will penetrate the thick clouds of emoji and cat GIFs that blanket the public's skull.

QUESTIONS? Send them to dr.know@wweek.com.