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Why Are Road Lines So Hard To See In The Rain?

Image by Bru-nO from Pixabay

You should be familiar with yellow and white lane markings directing motorists. While driving on a recent rainy night, WVXU's Tana Weingartner noticed she was having trouble seeing those lines. She started looking into the problem and quickly found she's not the only one, and it's not just a Tri-State problem.

It was dark, it was pouring, and I had no idea if I was driving in my lane. A few days later, the very question I was wondering popped up on the discussion website Reddit: Why have reflective lane lines become so hard to see in the rain? When I brought it up in a news meeting, another co-worker exclaimed "I thought it was just me!"

While it felt like a new phenomenon to me, it turns out people have been asking the question for years.

"Fifteen years ago we were saying it but we were just talking to our friends about it face-to-face," says AAA Driving School Manager Mike Belcuore. "Now we can post about it online; we can ask people around the country and world if they're seeing it, too."

The answer, he says, is scientific.

"If you have water in a plain glass jar and shine a light through it, the light refracts. That's exactly what the water on the roadways is doing with your headlights. The (reflecting agent) is trying to do its job in the lane lines, but the water affects that and it doesn't reflect back at you and your eyes. Instead, the rays are refracted and go other directions."

It's called "specular reflection." For a full physics lesson, check out this blog.

What About A New Paint Job?

Glass beads are the most popular reflective agent, or optic, used in road line paint. They come pre-mixed, or are added just after a line is painted. The beads catch the light from headlamps, illuminating the lines. Some paints and striping materials are more durable than others, like thermoplastics. The Ohio Department of Transportation (ODOT) and the Kentucky Transportation Cabinet (KYTC) use both.

ODOT is responsible for more than 19,000 miles of roadway. Spokesman Matt Brunning says re-striping is done as needed.

"Our roadways are inspected annually, but we also do regular inspections with our county forces. They're out on the roads every day... As they see things that need to be addressed, we certainly will add a work order in and address them."

Each municipality is responsible for maintaining its roads, so some areas may re-stripe annually while others may do so less frequently. A KYTC spokeswoman says Kentucky last year expanded its use of wider, more durable striping on the state's primary road system to improve visibility. Higher volume roads, like interstates, are more likely to be re-striped yearly, she says.

What 'Drives' Disintegration?

Besides rain, dirt and grime clog the rough road surface, affecting reflection, and traffic in general wears off the paint. Andrew Goodrich at 3M says snow plows wreak havoc, too.

"While the marking may still be white, more often than not all those glass beads will be scraped right off the top," Goodrich explains. "We look at whether or not agencies groove or inlay the pavement marking to protect that from those snow plows, which Ohio does not."

The good news is 3M makes a product with microcrystalline ceramic beads that works even when covered with water. It's been around for a few years but isn't widely adopted. It costs a bit more, and snow plows are still a problem. The beads have a refractive index of 2.4, meaning they're more effective at returning light to drivers' eyes than regular beads, which have a refractive index of 1.5 to 1.9.

KYTC says it has a few pilot projects using beads with the 2.4 index of refraction. Ohio has also used them in some construction zones, though Goodrich estimates it's less than 1% of applications.

One of Ohio's top suppliers, Ennis-Flint, offersa structured pavement marking material that creates peaks and valleys that force water away. Chief Technology Officer Paul Carlson suggests transportation agencies are finally becoming interested in addressing wet-night visibility thanks to automation and self-driving vehicles.

"Any vehicle that uses cameras to detect pavement markings for lane departure prevention purposes needs to be able to detect pavement markings in all conditions," he points out. "If you can't see it with your eyes, the camera is going to have a hard time seeing it as well."

That, he says, is pushing the industry to innovate.

Credit Courtesy of Ennis-Flint
Two examples of structured pavement markings that force water away from reflective optics.

How To Deal

But back to this feeling that it's harder now to see the lines in the rain than it used to be. Perhaps climate change is at play? More and heavier rains mean more standing water on the roads.

In the interest of being thorough, I also checked in with Dr. Kavitha Sivaraman at the Cincinnati Eye Institute.

"As we get older," she points out, "our pupils tend to be a smaller size - the pupils are what regulate the amount of light that gets into our eyes - so as those get smaller, less light gets in and it makes it harder to see in the dark because there's less ambient light."

Plus, once we hit our 40s, we also have fewer photoreceptors in our eyes (there are two types, it's the rods that affect our low-light vision) and our vision is slowly clouding (cataracts).

Mike Belcuore, the driving school manager, has some tips for when you can't make out the lines. Some are obvious: slow down, increase your following distance, and pull over if you need to. People often don't think about windshield wipers but it's important to make sure those are in good condition as well. He also recommends following in the tracks of the car in front of you, and driving in the center lane, if possible, because roads are slightly curved to disperse water.

Finally, if your wipers are on, turn on your headlights. You may not be able to see the road lines, but at least the rest of us will be able to see you. Plus, it's the law in Ohio, though not Kentucky or Indiana.

This article was first published on Jan. 13, 2020. 

Senior Editor and reporter at WVXU with more than 20 years experience in public radio; formerly news and public affairs producer with WMUB. Would really like to meet your dog.