Jackie from Longmont writes,“What's Driving You Crazy? Why do they put grooves in concrete roads like where I live on Highway 66 to Lyons and Highway 287? Seems like it not only throws the cars around but wears our tires out more quickly. I can’t see that it helps with ice or heavy rain — only makes it worse.”
The main reason for those lines Jackie is for water drainage. Concrete, for all practical purposes, is a non-porous substance when it comes to rainstorms, so the groves help disperse some of the rainwater that is on the road and helps it dry out faster. Asphalt, or blacktop, is more porous than concrete and has a higher capacity to absorb water, so there's no need for grooves.
According to an article in USA Today, the lines in concrete were first started as a solution to a problem NASA engineers needed when landing the space shuttle on a wet runway. It reads, “The requirements for landing space shuttles led NASA scientists to do extensive research on minimizing hydroplaning on runways. They discovered that cutting grooves into runways helps channel water away from the runway and significantly reduces accidents. Many highways and airports now have grooved pavement.”
I spoke to Leah deRiel, senior project engineer for the City of Miramar, Florida — which uses lots of concrete for roads and receives lots of rain — about the effectiveness of these lines.
She tells me concrete can't even come close to absorbing all the rain produced even by a small thunderstorm. She says road crews are supposed to pour a concrete roadway so there is a high point in the center. That slope allows the rainwater to flow to the side of the road.
“However, while you do want positive drainage and that general direction of flow, you don't really want that kind of water flow, a smooth sheet of water flowing across the road in front of you," she says. "We call it sheet flow, and it's really easy for cars to hydroplane on.”
deRiel says even though these longitudinal grooves present a minor obstacle to water flowing easily from the center of the road to the side of the road, what they also do is make the water turbulent, which helps with tire traction in a rainstorm.
“This is the same principle on tires and why it's so important to have good tread," she says. "Not only do the groves channel water, they help with traction by increasing force on the remaining area. There's a point at which the water will flow like a sheet anyway if you have high enough water volume, so there is a point at which the groves don't really help much, but they're way better than nothing.”
deRiel tells me the grooves should be cut nearly longitudinal to the driving surface.
“There may be a little bit of variation in this because the grinding machine is run by a human operator, but they’re not going to be an insuperable obstacle that sucks your vehicle across two lanes of traffic," she says. "They’re pretty small, and anyone operating a motor vehicle should be actively driving which means paying attention to road conditions, surrounding traffic, weather, etc., and actively steering. Winds in a thunderstorm can generate more force on a car than these grooves, and drivers have a responsibility to maintain control of their vehicles at all times.”
Another reason for the long lines in the concrete, according to deRiel, is to help with pavement joint impact reduction. This is why the grooves are cut longitudinally rather than transverse.
“Concrete pavement is usually poured in 12-foot-by-12-foot slabs, depending on how wide your lanes are," she explains. "Concrete is a brittle material. In order to distribute load evenly, slabs are poured as close to square as practical. They're also poured to get the joints as close to lane lines as possible, to minimize the occurrence of your tires riding over a longitudinal joint all the way to your destination. If you're doing 5,000 feet of pavement, that's a lot of joints. Imagine pouring, by hand, all of those slabs. What are the chances that they'll all be at exactly the same elevation? Not a snowball's chance in Miami. If you look at them from the roadway surface, you'll probably be able to see an elevation difference between two adjacent slabs. Get in a car and drive it, you'll be able to feel the difference in elevation... on every...single...slab. Cars don't like the bumps, drivers don't like the bumps, passengers don't like the bumps, and maintenance workers don't like the bumps either because bumps in pavement generate impact loading. Cars launch off one slab only to slam down on the generally unsupported edge of the next slab. This causes slabs to crack, generally near the joints, which is a maintenance nightmare. Keep in mind, water flows into and can wash away some of the subgrade below these slabs, especially at the joints. So, how do you fix this nasty little problem? You grind the top quarter inch of concrete using a machine, so all the slabs line up.”
As for the groves affecting the life of your tires, deRiel tells me, “Every driving surface has texture. Even ‘smooth’ driving surfaces have voids/grooves/irregularities when you look at them up close. For example, freshly paved asphalt roadway generally has some small voids that serve the same function as the grooves in concrete, they help with drainage and preventing sheet flow. Grooves in asphalt would be terrible from a materials standpoint. Asphalt isn't nearly as durable as concrete, so grooving in asphalt would result in one giant, continuous pothole. You know what surface doesn’t have texture and is pretty much as smooth as you can get? Ice. And while ice won’t wear out your tire, it’s not what you want to be driving on because it turns out that smooth, slick surfaces are unsafe. You want your tires to wear down over time. It’s the result of them doing what they’re supposed to do, which is to grip the road.”
Denver7 traffic anchor Jayson Luber says he has been covering Denver-metro traffic since Ben-Hur was driving a chariot. (We believe the actual number is over 25 years.) He's obsessed with letting viewers know what's happening on their drive and the best way to avoid the problems that spring up. Follow him on Facebook, Twitter or Instagram or listen to his Driving You Crazy podcast on iTunes , Stitcher , Google Play or Podbean.