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Originally published Monday, December 17, 2007 at 12:00 AM


Listen closely: Our roads are getting quieter

Highway 520's sweet spot — a sliver of freeway near the eastern end of the Evergreen Point Floating Bridge — is so smooth and...

Seattle Times Eastside bureau

Testing 2 kinds of quieter pavement

In the 1960s, when most of the state's freeways were built, Portland-cement concrete — a strong but noisy material — was the standard. Some roads have since been overlaid with conventional asphalt. Here's what the state is testing:

Asphalt rubber: Is standard asphalt with granular rubber particles used to bind the aggregate together. Since asphalt rubber is usually 15 to 20 percent air — compared with about 4 percent in conventional asphalt — it has more spaces to trap sound waves than conventional asphalt.

Polymer-modified asphalt: An asphalt that uses polymer, a synthetic fibrous material, instead of rubber to bind the aggregate. Like asphalt rubber, it contains a greater percentage of air voids than conventional asphalt, enhancing its ability to absorb sound.

Highway 520's sweet spot — a sliver of freeway near the eastern end of the Evergreen Point Floating Bridge — is so smooth and quiet even an ancient Geo Metro feels like a first-class cabin.

This is a test. This is only a test.

But to Walter Scott, those 2.25 miles between Hunts Point and Medina sound a lot like success.

Thanks in part to Scott's advocacy, the state Department of Transportation (DOT) is testing materials there designed to turn down the din of traffic.

The DOT calls them "quieter pavements."

Those who drive the highway or live nearby call it a huge improvement: "People who live along 520 are like, 'You are a godsend,' " Scott said.

Another test site is set up on Interstate 5 near Lynnwood, and next year workers will begin laying quieter-pavement sections on I-405 in Bellevue.

If the results are promising, Scott and others are hoping freeway noise will someday drop from a scream to a whisper.

The new materials, already being used in a half-dozen states and a handful of countries, contain tiny voids, like a Rice Krispies bar, that trap sound waves rather than letting them bounce around on the surface.

The tests will compare the new materials with existing ones to determine how long they would last, how much they would cost and how quiet they are.

Scott is optimistic.

Six years ago he sought a noise wall to block the constant drone at his Beaux Arts home from cars crossing the I-90 bridge. When that effort failed, he switched to ground-level technology and a grass-roots approach. Together with civic groups, city councils, state representatives and a host of individuals, he organized a coalition of advocates who asked Gov. Christine Gregoire to direct the DOT to test quieter pavements.

It worked.

Now when people assume residents of Hunts Point and Medina got a test section on 520 because they're rich, Scott is quick to respond.

"They got it because they asked for it," he says.

Noisy "tire slap"

Grab a friend. Go outside, stand 3 feet apart, and speak in normal tones.

If you can't understand each other, ambient noise has likely exceeded 66 decibels. When freeway sound gets that loud, the state will often try to mitigate it with noise walls, trees or earthen berms.

The problem is the pavement. "Tire slap" — the sound produced when rubber meets the road — accounts for about 70 percent of all noise on roads where speeds exceed 30 miles per hour.

When workers in Phoenix paved the city's potholes with crumbs of recycled tires in the 1960s, they accidentally discovered a way to decrease the impact of tire slap. The recycled rubber not only smoothed the streets, but it also dampened sound. Asphalt rubber was born.

Since 1988, the Arizona Department of Transportation has used asphalt rubber in more than 3,000 miles of pavement overlays. Arizona now recycles 70 percent of its used tires back into the highways, eating up about 1,500 tires per lane mile of highway.

Although the federal government doesn't recognize pavement types or textures as a form of noise mitigation, that hasn't kept asphalt rubber from oozing into California, Arizona, Texas, Florida, Alabama and Georgia.

Washington joined them in August 2006, paving 1.07 miles of asphalt rubber and an additional 0.77-mile strip of polymer-modified asphalt on southbound I-5 in Lynnwood, alongside conventional, dense-graded asphalt. In July of this year, the Washington state DOT installed similar test sections on Highway 520, between Hunts Point and Medina.

Next year, the agency will do more testing on I-405 in South Bellevue. That project will include 1.25 miles of rubberized asphalt and 1.25 miles of a polymer quiet pavement on northbound I-405 from Southeast Eighth Street to Coal Creek Parkway and southbound from I-90 to Coal Creek Parkway.

The biggest obstacle to quiet pavement's success here is the state's cool, wet climate, said Linda Pierce, the state's pavements engineer.

The extra air holes in asphalt rubber increase sound absorption but also let in more water and air. Pressure from air and water, both inside and outside the pavement, can loosen the aggregate.

Studded tires in the winter make it worse, picking away at the loose aggregate, Pierce said, so that "like a piece of wallpaper on a wall, you get one little piece of it and you can tear the whole thing down."

Earlier rubber trials

It's not the first time the DOT has put rubber in the road. In the 1990s, the agency added crumb rubber from recycled tires to a dense-graded asphalt south of Olympia to see whether it would cut down on cracking.

That material added 20 to 30 percent more to the cost, Pierce said, "and we weren't seeing a 20 to 30 percent improvement in pavement life."

Quieter pavements may be another story. The DOT is using a different mix of asphalt rubber with the goal of determining whether any of the new pavements are quiet enough, and last long enough, to justify the cost.

Pound for pound, asphalt rubber and polymer asphalt are more expensive than conventional asphalt, but since they're placed at half the thickness, they end up costing about the same. However, asphalt rubber's life span tends to be several years shorter, Pierce said.

To measure sound and compare results with those in other states, the DOT mounts a microphone on a rear wheel of a vehicle, about 2 inches above the pavement.

Recent tests show older asphalt registers about 105 decibels; brand-new conventional asphalt registers about 100 decibels, and new rubberized asphalt tends to be about 95 to 96 decibels.

States using asphalt rubber have reported it reduced road noise by nearly half. Recent test results on 520 showed new rubberized asphalt and a new polymer paving product also cut noise by about half, although road wear and weather could diminish this effect over time.

Shawn Gilbertson, the DOT acoustics special-studies manager, said his team is trying to get the best, most accurate data.

"Increasingly, WSDOT has to listen to what the public is saying," said Gilbertson, who acknowledges Scott's contribution to the push for quiet pavement. "Part of the reason we are looking into this technology is because people were asking us to do so."

If transportation corridors improve the environment and neighborhoods, people also may be more likely to support paying for them, an argument made by several state legislators in a letter to Gregoire last year urging tests of quieter pavement.

It will be five years or more before the DOT can tell how well the quieter pavements perform, but Pierce, the materials engineer, said there's reason to be hopeful. "I've spent the last nine months of my career on quiet pavement," she said. "This is what we do now."

Even if the tests are successful and the state finds a use for quieter pavement, repaving is an incremental process. It could be decades before it comes to a roadway near you.

The important thing, Scott said, is the state is taking a serious look at the new technology.

"WSDOT is now seen as the darling child for being innovative — and they should be," he said.

"It was the good idea that couldn't be killed."

Amy Roe: 206-464-3347 or

Copyright © 2007 The Seattle Times Company

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