Under its Quiet Pavement Pilot Program, Arizona can use its asphalt-rubber friction course, as a noise mitigation strategy and receive federal funding for highway construction. ADOT’s Delton said several rubberized asphalt overlays on concrete are about a decade old and show excellent performance when used that way. And he said that compared to a concrete top surface, an ARFC overlay provides a smoother ride. ARFC provides comparable friction levels to a concrete surface, reduces the amount of water spray, and provides increased contrast to pavement markings in wet weather, Delton said.

Furthermore, Arizona research shows that pavements with ARFC experience less rutting than overlays or inlays without rubberized asphalt. And, pavements with asphalt rubber as the wearing course cost less to maintain than neat asphalt cement surfaces, Delton said. ADOT data shows that most asphalt inlays and overlays will experience the first crack at about four years of age. But once initial cracking occurs, asphalt rubber greatly reduces the yearly increase in cracking.

The oldest section of ARFC in Arizona is an overlay of concrete pavement on Interstate 19 in Tucson. That section was the first of the modern ARFCs placed in one lift. Maintenance costs have been very low. Earlier ARFC layers were placed in three lifts.

"The I-19 section was placed in 1989, has received no major maintenance since then, and is still serving traffic today," Delton said. "Many years of performance history of ARFCs placed in Arizona have shown it to be a durable and long-lasting surface course. In virtually every performance measure, it does better than the comparable mix without rubber. This is the case when ARFC is used as an overlay on either asphalt or concrete pavements."

Participants in the European Scan Tour studied thin surface treatments in several countries; Colsoft, which uses a polymer-modified bitumen with a crumb rubber addition; single-layer porous asphalt surfaces; two-layer porous asphalt; and various concrete surfaces including exposed aggregate concrete.

Two-layer porous asphalt is in the experimental stage in Denmark, France, and Italy, said NAPA’s Newcomb. In the Netherlands, it’s in the developmental stage. A typical design in Denmark calls for a top course of 1 inch of 0.333- or 0.5-millimeter aggregate. The bottom layer consists of 1.75 inches of 0.6875-millimeter aggregate mix.

Noise reductions are significant with two-layer porous asphalts. They are 8 to 9 dB(A) quieter than conventional mixes, and 4 dB(A) quieter than single-layer porous mixes at high speeds, Newcomb said. On the downside, construction costs of two-layer mixes are 25 to 35% higher than a single-layer mix. Construction of the two-layer mix should be placed warm-on-warm. Tack coats are essential. Typical voids in the mix run greater than 20%, and typical binder percentages are 5.7 to 6%, based on aggregate weight.

Clearly, tire/pavement noise is an issue that has spread into urban areas across the country. What’s an agency to do? While the FHWA prohibits the use of pavement types or surface textures as a noise abatement measure, a Quiet Pavement Pilot Program should account for noise reduction benefits of pavement types, said FHWA’s Mark Ferroni, Highway Traffic Noise Team.

Perhaps NAPA’s Newcomb summed it up best: "Highway agencies should begin measuring the tire/pavement noise profile of their commonly used surfaces," he told attendees at Quiet Asphalt 2005. "Surfaces capable of reducing noise by 3 dBA(A) should be considered where noise is an issue."