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 and southbound 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 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 will lay 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.