Reading road signs at night is all about fonts and prismatic reflectivity (eh?)


If you are, like me, at an age where AARP solicitations make up half of your mail, then you’ve probably noticed that you can’t read road signs at night as easily as before.

Turns out it’s a pretty complicated problem, which is why it’s the subject of a giant experiment on Interstate 93 at Concord.

“There are several levels of road signs that are used, but to simplify the discussion, they can be grouped into three categories: engineered grade, high-intensity prismatic and micro-prismatic,” explains William Lambert, who was the head of state. traffic engineer for 15 years, explained the problem to me in an email – which, at the very least, shows that the phrase “to simplify the discussion” means something different to an engineer than to the most of us.

Lambert wrote to me because I had approached him on a related subject: the typefaces used to write road signs. This is also surprisingly complicated and became controversial last month after the federal government did an about-face on the subject.

Here’s the backstory, gleaned from various sources with Lambert’s help.

The standard traffic police, called Highway Gothic, have been around since at least the 1940s and were codified in the Uniform Traffic Control System Manual, the Federal Guide to Road and Highway Design in the United States

Why bother with fonts? Because it is difficult to create signs that can be read with precision by people speeding by in all kinds of conditions, especially signs that can be read easily both in the daytime, when illuminated mainly from above, and at night, when the lighting consists largely of light reflected from the moving headlights beneath them. The design of the fonts – the exact shape, thickness, and spacing of letters and words – plays an important role in getting this job done.

In 2004, the federal government replaced Highway Gothic as the recommended policy with Clearview, which looks a lot like laymen but has been modified to help older drivers at night.

In particular, the shape of several lowercase letters like “e”, “o” and “s” has been changed in Clearview to compensate for halo, a term for light propagating beyond expected limits to form fog or fog. visual blur. Older eyesight is particularly sensitive to the halo.

But then, after 12 years of using that standard, the U.S. Department of Transportation suddenly announced in February that it no longer liked Clearview and would not approve it for future panels. The resulting kerfuffle led me to contact Lambert to find out more.

The Clearview decision was made because real-world experience has shown it to be less readable, not more, on signs with dark letters on a light background, such as speed limit signs.

As for the road signs, which have light letters on a dark background, Clearview didn’t really help at night: Observer was the main determining factor in improving visibility and readability at night.

Now you understand why Lambert emailed me about reflectivity when I asked him about fonts.

Engineers realized long ago that words and signs need to reflect headlights to be readable, because you can’t put light on every freeway notice.

“The first technology was ‘cat eyes’ – reflective buttons in the print” that mimicked the shape of each letter inaccurately, Lambert said.

Decades ago it was replaced with glass beads embedded in leaves, which made the whole letter reflective and much more readable.

In the 1990s, “traffic sign makers came up with the Shiny Reflective Coating” that uses zillions of tiny little built-in prisms to better reflect light.

This stuff comes in two types. One is high intensity prismatic, which is more durable and brighter but reflects light like a mirror, bouncing it away from the direction it is coming in. (Remember “the angle of incidence equals the angle of reflection” from the physics class.)

The other type is micro-prismatic, which reflects about two-thirds of the light directly in the direction it came from – in this case, toward cars rather than upwards.

The micro-prismatic design looks brighter to drivers for obvious reasons, so we should be using it all the time, right? False – which brings us to the experience.

In the southbound lane of I-93 at Concord, two large signs hang from the I-393 bridge. One tells you that Exit 15-E takes you to Loudon and Portsmouth, the other tells you that Exit 14 takes you to Loudon Road.

The Exit 15-E panel has a micro-prismatic coating on the bottom of the panel and the letters. The Exit 14 panel has a micro-prism coating on the letters and a high intensity coating on the rest of the panel.

They were installed at the same time in what the tech world calls an A / B test, where two slightly different versions of the same product are created to see which performs better.

Lambert says it was not a contest. The sign with two different coatings is easier to read due to the contrast. This two-layered approach is now standard for all highway signs across the state.

I drove under these signs all the time and had vaguely noticed something different about them. After Lambert pointed it out, I traveled under them at sunset and it’s absolutely true: with the headlights on, the 15-E exit sign is more washed out and harder to read.

Alright, but what about the Feds switcheroo on highway police, the problem that got me calling Lambert in the first place?

That’s not a problem for New Hampshire, Lambert said, because we never adopted Clearview for our road signs in the first place.

Highway Gothic is free but Clearview is a private and proprietary font: to use it, you would have to purchase the license. The cost is minimal – $ 759 for the entire 13 font family per municipality – but as you can imagine the idea of ​​buying something we could get for free didn’t work out well, so we’re stayed with Highway Gothic.

We are not alone. Almost half of the states have never adopted Clearview, so road signs across the country are a mix of Clearview and Highway Gothic, although most of us don’t even notice it.

By the way, neither in 2004 nor today did the federal government’s decision require modification of existing signs. These standards only apply to future brands, and not retroactively.

As for me, the next time I can read an exit sign at night without squinting too much, I’ll give thanks for the micro-prism and high-intensity prism contrast.

I hope I don’t do it out loud, otherwise my passengers will get very worried.

(David Brooks can be reached at 369-3313, [email protected], or on Twitter @GraniteGeek.)

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