Granite Geek: I-93 Experiment Focuses On Reading Highway Signs At Night

Feb 17, 2016

You may not have put much thought into the design of the signs on the highway, but right now engineers in New Hampshire are giving careful to how these signs reflect light. An experiment on Interstate 93 is comparing two different kinds of reflectivity to find out which is easier to read at night. Granite Geek David Brooks spoke with NHPR's Peter Biello.

This can get complicated, especially if you are not an engineer, so let’s try to simplify this first. There are two types of reflectivity that state engineers are testing on signs on I-93 in Concord. One is called High-Intensity Prismatic and the other is micro-Prismatic. So, first tell us which High-Intensity Prismatic is.

Well, both of them are ways in which basically sign manufacturers imbed lots of little reflective stuff into the sign.

Both the letters and the background?

Well, anything it could be. So, high-intensity and micro-prismatic—I love the name micro-prismatic by the way, I think I used it eight times in my column—high-intensity prismatic and micro-prismatic, they have different shapes and distributions of this reflective stuff. High-intensity reflects a lot of light and it reflects it just like a mirror does. So, if you—you know, light comes onto a mirror, it bounces in the opposite direction, right? Micro-prismatic reflects it back at you, so light comes into your mirror as if the mirror shined it right back at you, which it doesn’t normally do.

So, micro-prismatic does not reflect as much light, but it reflects it in a different way. So, the question is, which one of these ways is better for a sign for people reading it at night at high speeds when they’re not paying much attention and they’re zipping along and they suddenly have to change lanes because their exits coming up?

Okay, so how did the engineers set up their test?

It’s your standard A/B test, as any technology person will be familiar with. You have two different things which are slightly different, you put them both out there into the wild, and you see which one behaves better, basically. It’s done with software all the time. It’s done less often with material objects, because they’re harder to make. These signs are, I don’t know, 60 feet across, and enormous and expensive, so it’s not like you can go zip them up all the time.

But they put these up, they’ve actually been up a couple of years. And I didn’t even know about it, I just sort of stumbled into this story by accident. And one of them—so this is at exit, this is on the I-393 bridge as I-93 is going south right through Concord—and one of them which tells you the next exit, which it goes to Laconia, has Hi Intensity Prismatic on the sign, but micro-Prismatic on the letters. So, the sign shines in all directions, and the letters shine right back at you. And the other one has micro-Prismatic on both of them.

So, they’re side by side, one has different kinds of reflectivity, background versus letters, the other has it on both background and letters— which one seems to be working better for people trying to see it at night?

Well, you know I’ve driven under these signs a lot, and you know I’ve sort of vaguely noticed they were somewhat different, but I can’t say I’ve ever said, oh my gosh look at that. But I drove under them after learning about this from William Lambert, who is the state’s traffic engineer and has been for quite a while, and it’s quite obvious that the one on the right, which is the one that has the same reflectivity on both sign and letters is not as easy to read, the letters sort of wash out. Particularly for those of us who have a certain, you know, we’re reaching our golden years and our eyes aren’t quite what they once were.

At nighttime, vision, there’s a thing called halation, which is where light sort of turns into a bit of a blur and makes it a little harder to distinguish whether a letter is an O, or an E, or an S. So, that’s the thing they’re trying to handle. And it’s quite obvious that if you have two different kinds of reflectivity, then it’s easier to read.

So, is the state going to now to have to change all the signs that exist in the state to make them work in the same way that the sign that reflects light better works?

No, most of them are like that already. This was a test to see, gee should we change, should we, you know. Future signs, it’s not often that anything like this requires you to go in and change all existing signs, because that’s expensive. But they are frequently doing, doing tests all the time to figure out what’s the best sign design for future signs, because obviously it’s important, you know? If people aren’t paying attention because they’re trying to read the sign, they’ll get into an accident.

And what about font? You were investigating font when you discovered this issue of reflectivity.

Exactly. Fonts like, you know, fonts on your computer, the shape of the letters. So the government, the federal government that is, had standards for a certain kind of font on highway signs for decades— many, many decade, back at least to the forties— called Highway Gothic. And about a decade ago, twelve years ago actually, they said, you know what this other font called Clearview is actually better, and it’s specifically better for older people at night. Reflected light. So, future signs, anybody doing them, you should probably do them in Clearview. Now, it turns out New Hampshire never went to Clearview, because,  for a good New Hampshire reason, Clearview costs money.

Too expensive.

Too expensive. Which is not much at all, but still something, whereas Highway Gothic is free. So, we stuck with the free font and good thing we did, because 12 years later, just last February actually, a month ago, the federal government said, you know what, we’ve been studying this further, and Clearview actually isn’t better at night, and in certain other circumstances for signs that have dark letters on a light background like a speed limit sign, Clearview is actually slightly worse. So, they said, so stop using Clearview for future signs. So, we’re okay for fonts as it is.