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Common Questions

Why are we studying quieter pavement?
Residents throughout the state have told us they want solutions that will reduce traffic noise experienced at their homes.  We are testing quieter pavement as a potential tool for reducing road noise on three of Western Washington's busiest highways. 

The challenge is to design pavements that reduce noise, while still providing durable, smooth and safe surfaces.  We are studying a new generation of quieter pavements to test their durability against ever-changing driving conditions that include heavy rain, snow, ice and studded tires.  We also want to know how long their noise-reduction abilities last and how much noise they will eliminate.
What are the types of quieter pavement?
We are testing two types of quieter asphalt and several types of quieter concrete.

Quieter asphalt – known as open graded friction course pavement:

  • Asphalt modified with a polymer
  • Asphalt modified with rubber

Quieter concrete – made quieter by using different texturing techniques. 

  • Longitudinal tining – creates shallow channels in the concrete using a rake.  Longitudinal tining is different than traditional transverse tining because the channels are made in the direction of traffic flow.
  • Diamond and whisper grinding – used on existing concrete.  Crews use diamond saw blades to remove a thin layer of hardened concrete creating a texture pattern similar to corduroy.
  • Dragging –  includes carpet, burlap and Astroturf™ dragging.  Crews drag an inverted section of artificial turf or other material behind the paving machine. 

How does quieter pavement work?
Quieter pavement has a different surface texture designed to absorb noise created as tires roll across the road surface.  Traditional pavements have positive texture – small bumps sticking out of the pavement – which are not very effective in reducing noise.  The compounds in quieter asphalts create negative texture – air pockets in the pavement that reduce noise.  Concrete is made quieter by using different texturing techniques on the surface of existing or new concrete.

How are we studying quieter pavement?
We are studying several qualities of the pavement:

  • durability - including skid resistance, smoothness, cracking and studded tire damage
  • quality and quantity of noise reduction
  • length of noise suppression over time

We will study the durability of the pavement using laser levels and high speed cameras to measure the amount of wear and tear on the roadway.  To measure the amount of noise reduction, crews will use special microphones placed on the side of the freeway as well as microphones attached inside and outside of vehicles.  We will measure noise levels before the pavement is installed and monthly after the pavement is installed.

What is traffic noise and how do we measure it?
The amount of traffic noise depends on traffic volume, speed, and the type of vehicle.  Generally, an increase in volume, speed, or vehicle size increases traffic noise levels.  Vehicle noise is a combination of sounds from the engine, exhaust and tires.  Other conditions affecting traffic noise include defective mufflers, steep grades, terrain, vegetation, distance from the roadway, and shielding by barriers and buildings.

We have three main methods for measuring pavement noise levels in the field:

  • Statistical Pass-by Procedure - We place microphones at a defined height and distance from the vehicle path at the side of the roadway.
  • Single Vehicle Pass-by Method - We measure noise from cars and light trucks at a specially designed site.  The vehicle approaches the site at a precise speed and gear.  A sound level meter is set at a specified distance from the center of the travel path and captures the sound level of the vehicle as it passes.
  • Near-field Techniques - Near-field techniques, such as the on board sound intensity method, measure sound pressure using microphones mounted on the vehicle near the vehicle tire.

Where are we putting down quieter pavement?
We chose test locations on three of western Washington's busiest highways:

  • We are testing nearly one mile of rubberized quieter pavement and nearly one mile of polymer-modified quieter pavement on southbound I-5 between Swamp Creek Interchange (I-5/I-405/SR 525) and the 44th Avenue West Interchange in Lynnwood.
  • We are testing 1.25 miles of rubberized quieter pavement and 1.25 miles of polymer-modified quieter pavement on northbound I-405 four miles south of downtown Bellevue.  We will also test nearly one mile of quieter concrete.  We will use the whisper grinding technique on a strech of existing concrete. 
  • We are testing .5 miles of rubberized quieter pavement and .5 miles of polymer-modified quieter pavement on both directions of SR 520 in the Medina, Clyde Hill, Hunts Point, Yarrow Point and Bellevue area.
  • We are also testing quieter concrete (pdf 791k) on I-90 in Spokane, on I-205 east of Vancouver, and on I-5 in Federal Way and south of Tukwila.

How did we select the study locations?
We chose these locations on I-5, I-405, and SR 520 because they are representative of high traffic, urban locations where quieter pavements would most likely be used.  We chose the quieter concrete test locations because these projects involved installing new concrete, and gave us the opportunity to test new techniques.

How does quieter pavement compare to traditional pavement?
The most common road surfaces today are Portland cement concrete  pavement and hot-mix asphalt pavement.

  • Durability – It is important to evaluate pavement durability because pavements that are less durable need to be replaced more frequently, which increases highway maintenance costs.  Traditional asphalt has an average lifespan of about 16 years in Western Washington.  Traditional concrete can last up to 35 years.   In other states, quieter pavement has an average lifespan of 8-12 years.  
  • Cost (as of July 2006)
    • Standard (hot mix) asphalt - $58,900 per lane mile
    • Polymer-modified quieter asphalt - $81,600 per lane mile
    • Rubberized quieter asphalt - $93,200 per lane mile
  • Noise levels – Generally, hot mix asphalt pavements are quieter than Portland cement concrete pavements and open graded friction course pavements are quieter still.  To be useful as a noise reduction tool, however, the pavement must continue to significantly reduce noise over its lifetime. 

Where else is quieter pavement used?
Quieter asphalt has been used in Arizona, California, Texas, Georgia, Alabama and Florida.  Other states using quieter asphalt report an initial noise reduction of 3-5 decibels.  A 3-decible change is roughly equivalent to cutting traffic volume in half, whether it be from 2,000 to 1,000 vehicles, or 200,000 to 100,000 vehicles.

States currently using OGFC
States shown in dark gray are currently using quieter asphalt pavements.

Have we used it in Washington State before?
We tested earlier generations of quieter asphalt on I-5 in western Washington in the 1990s.  These projects deteriorated rapidly and developed ruts due to studded tire wear.  The average lifespan was just four to eight years before the roadway had to be replaced.

We have not tested quieter concrete in Washington State previously.

What are the challenges of installing quieter pavement in Washington State? 
Quieter asphalt has been most successful in warmer, drier states.  For instance, Arizona's Department of Transportation requires that the existing pavement surface be at 85°F before installing rubberized pavement.  In western Washington, where paving is typically done at night to avoid major traffic backups, the existing pavement surface temperature rarely reaches 75°F – even in the warmest of summers. 

To develop specifications for testing quieter pavement in western Washington, we looked at the installation guidelines for other states, and consulted the Rubber Pavement Association.  On southbound I-5 in Lynnwood, we required that the air temperature be at least 55°F.

On I-405 and SR 520 we installed down quieter pavement during the day.  We want to know if the temperature during paving will affect the pavement's durability and noise reduction capabilities.  On all three highways, we are working with quieter pavement experts from Arizona during paving to get the best trial sections possible.

There are no known challenges for installing quieter concrete.

How long is WSDOT testing each section of quieter pavement?
We will study each test section of quieter pavement for at least five years or until they reach the end of their useful lives. Our engineers need several years worth of data to observe trends relating to durability and noise reduction. Our engineers will continue to take measurements until they've determined the quieter pavement has reached the end of its useful life. We will measure several characteristics:

  • durability
  • quality and quantity of noise reduction
  • how noise suppression abilities perform over time
  • These tests will provide us with the data we need to determine if quieter pavement is a good choice for Washington highways.

What is a pavement’s “useful life”?

  • A pavement’s useful life is near the end or over when:
  • Significant rutting occurs (10 mm deep or more)
  • The pavement cracks causing the roadway to become potentially hazardous for drivers.

The life span of WSDOT’s standard hot mix asphalt is about 16 years.

It’s been at least two years since WSDOT installed the last section of quieter pavement. How are the two test sections performing?

  • Our quieter pavement test sections are not performing as well as we would have liked. Any audible noise reduction was lost with in the first six months of paving.
  • Additionally, winter's storms in late 2008 significantly affected the durability and noise reduction capabilities of our test sections. The freeze thaw cycle, use of snow chains, sanding and frequent plowing caused the pavement on both I-5 and SR 520 to break apart. Both test sections will need to be replaced with standard hot mix asphalt within the next 2 -3 years.

Why are the two existing test sections not performing well?

  • Noise reduction: Our data shows that new quieter pavement was never audibly quieter than the standard hot mix asphalt normally used to repave our highways, and today the quieter pavement is actually registering louder than sections paved with just the standard mix.
  • Durability: The freeze thaw cycle, use of snow chains, sanding and frequent plowing caused the pavement on both I-5 and SR 520 to break apart. Both test sections will need to be replaced with standard hot mix asphalt within the next 2 -3 years.

If the quieter pavement on I-5 and SR 520 is not performing up to WSDOT expectations, why are you installing additional test sections on I-405

Our engineers still need more data before concluding the quieter pavement test program. This August, WSDOT contractor crews will install the third and final quieter pavement test section on northbound I-405 through Bellevue.

Engineers believe several factors make the test on I-405 in Bellevue unique:

  • Temperature: Quieter pavements have proven successful in states with warm climates such as Arizona, California, Texas, Georgia, Alabama and Florida. Engineers believe that the surface and air temperature during paving may affect the long-term durability and noise reduction abilities of the quieter pavement. To test this theory, WSDOT crews will install the quieter pavement test section in Bellevue during the day in August when temperatures in the Puget Sound are the warmest.
  • Paving over concrete: The final quieter pavement test section will be installed over existing concrete. WSDOT engineers want to know if installing quieter pavement over a ridged concrete base will affect the pavement’s durability and noise reduction capability.

What’s next for the quieter pavement program?

  • We will continue to study both quieter pavement test sections for five years or until they reach the end of their useful life.
  • We also see value in continuing to test quieter pavement on I-405. The I-405 test section is unique because we are paving 100 percent of the quieter pavement during the hottest days in August, and installing it over concrete. Engineers believe that temperature and the base on which the quieter pavement is installed may affect its durability and noise reduction capabilities. Tests in other states show that warmer weather allows the quieter pavement to set-up and stabilize better while it is being put down rather than paving it during the cooler Puget Sound evenings.
  • We are committed to reducing traffic noise for our highway neighbors in a financially and legally responsible way when new projects make it possible. We will continue to look into other quieter pavement and traffic noise reduction methods used in other states and around the world to help better the communities in which we live, work and play.

What is the funding source to replace a failing section of quieter pavement test with new asphalt?
We have made a commitment the federal government to study each quieter pavement test section for at least five years or until it reached the end of its useful life. By agreeing to these terms WSDOT can use pre-existing federal funds to replace section of quieter pavement that has failed.