Chet Skwarcan is an award-winning engineer, writer, and industry innovator in the field of traffic engineering. He is known for leveraging his creativity, logic, and technology to solve today’s engineering challenges.

Beyond the Nutshell

Perhaps you’ve noticed some traffic signals are sometimes completely “dark.” And they remain so until you arrive within a prescribed distance. And then suddenly they *illuminate*. How do they know?

I imagine you have discussed this conundrum countless times around the dinner table — and that is also likely the reason no one visits you anymore. But, in a nutshell, you’ll typically find nuts — unless you’re sitting near my friend Bob, in which case you’ll find nothing (except Bob). But, beyond the nutshell…well, that’s where this gets interesting…

The particular type of traffic signal is actually near my house. And when I say “near my house” I am speaking historically — because I sold that house (along with my accordion) when my monkey died (subject for future column).

That being said, this particular signal, and thousands exactly like it, are optically programmable. In layman’s terms: sometimes you can see it and sometimes you can’t. This hi-tech lens (invented in 1887) can actually be *programmed* to be visible from a predetermined range of viewing distances (PRoVD). Outside of this range, the signal appears dark to the human eye (not sure about other people’s eyes). And here’s the amazing part, the specific range of visibility is calibrated by applying duct tape to the obverse side of the lens (you’ll need a ladder). Yep, that’s right, I said a ladder.

Now a trained traffic signal technician applies this duct tape by opening the traffic signal indication gizmo. By looking through the signal lens in an obverse fashion, whatever traffic he *sees* is what *sees* him. This usually makes him chuckle. Here he is, up on a ladder, in the middle of a busy intersection — is he nuts? He smiles an obverse smile and tries to think about other things…

Anyway, he applies the duct tape to “limit” the view of the signal’s visibility to only those vehicles as predetermined by a professional traffic engineer (PTE). Now, should this cause confusion or accidents, well then, a professional traffic engineer was not involved and the technician decided, on his own volition, to make changes to the traffic signal. In fact, I have no idea what you’re talking about. I don’t even own an accordion (any more).

But, if a PTE specifies an optically programmable signal head, there is a common scenario where its application is quite appropriate. That scenario occurs when two signalized intersections are so close that signal indications for one could confuse vehicles waiting at the other (or vice-versa). There are also situations where turning vehicles could be misdirected, particularly when the driver sees signal indications intended for other approaches before seeing the signal indications for their own approach.

A final word of warning — these signals can create driver confusion. Because the signal initially appears dark, it could be interpreted as a malfunction. So proceed slowly, the signal may indeed show an indication. And if it does not, this is known as, The law of bulb-life exceededness.

Chet Skwarcan (traffic engineer, author, unique insights) with over 25 years of traffic engineering experience, solving (or preventing) traffic problems every chance he gets. He can be reached at