Highway noise barriers are a pretty common blight on American interstates, there to protect nearby homes and businesses from the motoring sounds of cars and trucks whizzing by. But what if I told you they don’t shield noise all that well, and may even amplify it?
In theory, noise barriers seem to make sense, since constructing a giant concrete wall between the offending roadway and the property nearby does block some noise just beyond the wall. But acoustics experts say that the problems start after that.
Have a look at this graphic produced by the Wisconsin Department of Transportation and dug up by Undark in their recent(ish) deep dive on the subject:
That shows what you might assume, that the greatest beficiaries of the noise barriers are those that are closest to them. More interesting, Undark found, is that, on top of not decreasing noise the farther you live away, the noise barriers might also amplify sounds in those areas as well.
But because noise travels in waves, not straight lines, sounds can and do go over the walls. This is why even with barriers standing 16 feet, homes several blocks away can hear the highway. Part of the sound wave is absorbed, part is reflected away from the wall, and part is transmitted through, [Mariano Berrios, environmental programs coordinator at the Florida Department of Transportation] explained. “Most of it goes above the barrier and gets diffracted, and gets to the receiver,” — that is, to a resident’s ears — he said.
This is especially problematic during certain weather conditions. When the consulting firm Bowlby & Associates, in Franklin, Tennessee, measured sounds around a highway in a yet-to-be-published study, they found that residents hundreds of feet from the highway could hear sounds some 5 decibels louder if the wind was blowing towards them, said Darlene D. Reiter, the firm’s president.
Weather, however, isn’t taken into account by the regulations. The noise model “assumes neutral conditions — no wind and no temperature effects — when in reality that happens only occasionally,” Reiter said. In the early morning, if the ground is cool but the air warms up, for instance, sound that would normally be pushed up is refracted downward, causing homes some 500 or 1,000 feet from the road to hear it loudly.
Those living up on hills or near freeway openings sometimes find the noise actually worsens once walls are built nearby. It was a gap in the barrier near his suburban New Orleans home — partially to accommodate a highway exit — that substantially increased noise in the backyard of attorney Harry Molaison. Although his house is roughly 500 feet from the service road leading to the interstate, “you have all this rebounding sound from one parallel wall to another,” he said.
The barriers are largely made of concrete, and cost over $2 million per mile, with some $6 billion spent on them through 2013. For me, though, the most astounding figure in the story is that some walls cost up to $92,000 per impacted home. In many places in this country $92,000 will get you a home or three.
Why do states keep building the barriers? Because the Federal Highway Administration says that states “must implement” noise abatement measures “where reasonable,” and the only noise abatement measures that the feds will fund are noise barriers, even as experts have long said that quieter pavements would be a better use of funds.
Here’s a short video showing how big a difference even a slightly quieter pavement can make:
Pavements are just one possible solution, though. Here’s a different one called 4Silence, offered by a commenter and developed in the Netherlands. It claims to solve noise pollution through slits along the side of the road:
Here’s a third solution, which is a wall and ugly as hell but also is an absorptive wall for sound, not a reflective one like concrete or metal.
There are no shortage of alternatives, in other words, even if, as it is, the cost structure will continue to incentivize the dumbest of them.
I suppose it should also be said that if you choose to live next to an interstate, you probably should expect some noise, though not everyone gets to choose where they live, and the ubiquity of highways in cities means that they can sometimes be unavoidable. And while electric cars are generally quieter than gasoline-powered cars, it’s the tires that make the most noise on highways, not the powertrain, meaning our electric future will be nearly as loud.
The answer? The answer, as always, is mass transportation, which we here in New York City have figured out, nay, perfected.