Earth Notes: Quieter Pavement
Excessive road noise is a scourge of modern life. Barrier walls along busy highways don’t always reduce the ruckus because sound waves can jump the walls—especially on windy days.
But a new paving method is helping muffle the sounds of traffic, and Arizona has been using it in several places around the state.
Vehicles generate sound in three main ways—from engines, tail pipes, and the whoosh of displaced air, especially tire-on-road contact.
As tires roll over pavement, small lumps and bumps on both surfaces cause vibrations, radiating sound from treads and sidewalls—worse from knobby tires on off-road and heavy commercial vehicles.
There’s also a clapping sound as air trapped inside the tire’s tread slaps the road. Stiff surfaces increase this effect—you can distinctly hear it when you cross from asphalt to concrete pavement.
Nearly half of all vehicles would have to be hybrids to noticeably reduce noise on city streets. Short of that happening, the Arizona Department of Transportation has for several years been resurfacing some state highways with rubberized asphalt.
Made with a mix of asphalt and “crumb rubber" from ground up tires, this more flexible paving surface reduces sound by an average of four decibels.
Rubberized asphalt has the added benefit of recycling around 1,500 discarded tires for every lane mile that’s paved with it.
Despite these innovations, the Federal Highway Administration will only fund new noise-reducing road surfaces if they’ve been evaluated for long term durability. So at least for now it’s individual states like Arizona that are spearheading the spread of quieter roads.