Executive Summary: Introduction

In the United States, the primary noise mitigation strategy for highway operations has been construction of noise barrier walls. However, these sound walls are expensive to build (often $1 million to $2 million per mile) and expensive to maintain. Graffiti is a major maintenance issue for highway personnel, prompting frequent complaints and requiring the redirection of precious transportation resources. In addition, the noise benefit of barrier walls is limited, often to less than 400 meters (m) from the roadway.

Prompted in part by the increasing cost of and concern about the effectiveness of sound walls, a growing number of engineers have turned their attention to attacking the noise problem at its source. Source control strategies include quieter vehicles and tires, speed control, additional building insulation, more aggressive building codes for new construction, zoning or right-of-way purchase, and quiet pavements. Many of these alternatives have been investigated with mixed results. For example, new tire designs have produced quieter tires, but the trend toward wider tires for improved handling and skid resistance has essentially negated this noise-reduction benefit. Limited useof quiet pavement in the United States has shownthat this approach has potential. Quiet pavement has alonger history in Europe, and the experience of highway agencies there is valuable in determining the benefit of quiet pavements as the preferred alternative in reducing highway noise. The objective of the quiet pavements scan team was to visit countries with the most experience in quiet pavement technology and learn from their experience.

The systematic reduction of noise associated with roadway operations has been a critical issue in Europe for more than 20 years. Countries in the European Union (EU) have agreed to map noise contours along all existing roadways by 2007. These maps will be made available to the public. Each country will develop an action plan to address problems identified in the noise map. Most countries have aggressive policy directives to limit noise along newly constructed facilities. In many EU countries, new quiet pavement alternatives are being used as one of the technologies to address noise problems.

The quiet pavements scan team was composed of a cross section of State, Federal, academic, and industry representatives. The team visited five countries over a 17-day period. The study design was based on a comprehensive desk scan of published research summarizing where the technology was most used, where it was first used, and where innovation was still being explored. Although the team had several countries from which to choose, it selected five that it visited in the following order: Denmark, the Netherlands, France, Italy, and the United Kingdom. This sequence was established in an effort to reduce travel time, maximize meeting time with experts, and visit field sites in each country. While in transit from the Netherlands to France, six team members visited several sites in Belgium. A summary of the visit to Belgium is included in Appendix A.

Significant Findings

Although the full report explores in detail the many significant findings by the team, the following were deemed to be of high general interest to executive or policy implementation readers:

1. Policy

   Highway pavement noise has been studied in Europe for more than two decades. Policies have been developed to mitigate noise through an integrated approach that encourages use of quieter pavements. All of the countries the team visited have implemented policiesthat require consideration of quiet pavement where noise is anticipated to be a concern.

   In addition, on June 25, 2002, EU implemented a significant Environmental Noise Directive that requires all member countries to do the following:

   • Determine exposure to environmental noise through noise mapping, including rural areas.
   • Use uniform prediction methods of assessment common to the members.
   • Ensure that information on environmental noise is made available to the public.
   • Adopt action plans based on noise-mapping results, with a view toward preventing and reducing environmental noise.

   The directive requires all member countries to complete strategic noise maps and adopt action plans by June 30, 2007.

   As is often the case in the United States, implementation budgets for the countries visited were much smaller than deemed necessary to implement the policy directive totally. The primary implementation funds were carved out of the existing construction budget, but a designated funding source did add status to the policy and direction to the program. The quieter surfacing costs were about 10 to 25 percent more than traditional surfacing.

2. Design

   The focus of the European effort is contained in three major quiet pavement technologies: thin-surfaced, negatively textured gap-graded asphalt mixes (such as Novachip, microsurfacing, and some stone mastic asphalt (SMA); single- and double-layer highly porous asphalt mixes (greater than 18 percent voids); and exposed aggregate concrete (EAC) pavements. The emerging trend is to use the thin-surfaced, gap-graded mixes with small aggregate in urban areas and areas subject to severe winter snow and ice accumulations. More porous gap-graded asphalt surfaces are used on rural and high-speed facilities with moderate winter conditions. EAC can be used where concrete pavement surfacing is allowed. Many highway projects are specified using performance specifications and are selected using best-value contracting methods. In many cases, pavement vendors respond to agency performance criteria with innovative solutions that often carry unequal risk, but if found effective, can be held proprietary for future project applications.

3. Noise Analysis

   The source level of quiet pavements is being incorporated into existing highway noise prediction models using varying methodologies. HARMONOISE (Harmonised, Accurate, and Reliable Prediction Methods for the EU Directive on the Assessment and Management of Environmental Noise), the common EU model being developed, will incorporate pavement type in the prediction, along with other advanced prediction parameters such as meteorological effects. To determine the noise benefit of pavements, most countries use multiple methods, including statistical pass-by (SPB) (ISO 11819-1), close proximity (CPX) (ISO 11819-2), and various controlled pass-by (CPB) methods, along with pavement sound absorption measurements. Each method has different strengths. In terms of vehicle types, the influence of quiet pavements on heavy vehicles is less well understood than for light vehicles; this topic is being investigated. Pavement noise benefits of as little as 2 decibels (dB) are being used in integrated noise strategies.

4. Construction

   Normal construction equipment and technology are used to construct quiet pavements. Porous asphalt (PA) mixes are used only on pavements that are structurally sound. Other defects in the underlying pavement must be minimal. Vehicle spray reduction and improved skid resistance are the two main reasons that porous surfaces were first used in each of the five countries. Noise reduction was a side benefit in the effort to produce a safer pavement during wet weather conditions. Contrary to normal practice in the United States, factors other than low bid are considered when awarding pavement construction contracts. Also, a contractor warranty of at least 3 years is typically included in the contract.

5. Maintenance

   Minor disagreements persist about effective maintenance of these negatively textured and often highly porous pavements. Although some countries require pressure washing and vacuuming of the pavements at least twice a year, other countries contend that the practice may not only be useless, but perhaps even harmful. The team was unable to discover any reliable data to substantiate either claim. Winter maintenance remains a challenge, especially on the highly porous pavements. Winter maintenance relies on advanced use of prewetted salt to fight formation of black ice on the highly porous pavements, resulting in a winter maintenance cost increase of 25 to 50 percent. Some countries have stopped using highly porous pavements in snow and ice regions, and instead are using SMA-type pavements with small aggregate.

6. Research

   Perhaps the most impressive finding of the team relates to the extensive amount of research on quiet pavement technology underway in the countries visited, including Roads to the Future (RTF); Silent Roads for Urban and Extra-Urban Use (SIRUUS); Program of Research, Experimentation, and Innovation in Land Transport (PREDIT); Sustainable Road Surfaces for Traffic Noise Control (SILVIA); and HARMONOISE. It was obvious that research is a vital part of the European culture. Governments conduct much of this research in partnership with industry, and have complex relationships with private entities to fund far-reaching research objectives. For example, under the SIRUUS program, companies are encouraged to submit innovative ideas that are judged by a panel of topical experts, and the best ideas are constructed as experimental sections. Selection of the experimental idea is a highly sought-after award and is often used as a marketing tool for other company products and services.

Implementation Recommendations

The team identified a significant number of implementation recommendations, and then categorized them into short- and long-term proposals. Following are some of the recommendations for immediate implementation:

1. The European experience demonstrates that porous mixes are effective in reducing noise when used properly. Early evaluation results in Europe indicate that two-layer porous asphalt (TLPA) appears to have potential application on high-speed facilities and produces exceptionally quiet pavements. Thus, this system appears to merit additional evaluation and research in the United States. Porous mixes should not be placed in urban areas where the operating speed drops below 72 kilometers per hour (km/h) (45 miles per hour (mi/h), since highly porous mixes tend to clog under slow traffic.
2. For an immediate improvement in the noise-reducing properties of mixes, a reduction in aggregate size in the wearing surface should be considered. In Europe, the aggregate sizes for quiet surfacing mixes are 0/4 millimeters (mm) through 0/10 mm. Since most State departments of transportation (DOTs) use the Superpave aggregate gradings of 19 mm, 12.5 mm, or 9.5 mm, a drop in routine aggregate mix size to the next smallest gradation is recommended and should produce a noise reduction of 1 to 3 dB.
3. Thin-textured surfacings using a small aggregate size are recommended for urban or low-speed sections. To achieve noise reduction, texture should always be negative (pavement depressions). Positively textured pavements such as chip seals increase noise.
4. Diamond-grinding blade configurations should be investigated and optimized to enhance noise-reducing properties of existing concrete surfaces in noise-sensitive locations.
5. EAC pavements should be researched further and considered when constructing new concrete pavements.
6. A team of acoustical experts and pavement engineering personnel should begin the process of developing American Association of State Highway and Transportation Officials (AASHTO) protocols for measuring the acoustical performance of quiet pavements. These protocols should capitalize on the extensive work completed and ongoing in Europe, as well as other locations throughout the world. Until new standards have been developed and adopted, State DOTs should use SPB (ISO 11819-1), CPX (ISO 11819-2), and various CPB methods to monitor existing pavement noise.
7. Consider updating the current noise policy and traffic noise models to take advantage of the benefits of quiet pavement technology through an integrated approach with other noise mitigation alternatives.