Each day communities around the globe are exposed to it. Just what it is and how much of it people should have to put up with is subjective and controversial. In some places people just deal with it, and in other places great care is taken to reduce or eliminate it. The biggest problem with noise is that it is only a problem when it is heard. Noise happens all around but no one cares until it affects them. In fact, very few people challenged to solve noise problems ever actually hear it. Problem solvers just deal with numbers on a noise meter or in a page of a report.
For decades, the solution to highway noise problems was to block it out with barriers and walls. Highways are significant contributors to noise where we work and live. As highway traffic increases in both speed and volume, users and neighbors alike are exposed to increasing levels of noise. Walls have been built, but the noise is still a problem. The walls can be a problem, too. Sound walls are expensive to build (often $1-2 million per mile or $20/sq ft) and expensive to maintain. The large walls attract graffiti and can require continued maintenance obligations. Walls also are not effective beyond the first row of houses and lose the reducing effect for homes and businesses 400 meters and beyond. More options are needed.
One option is to turn down the traffic noise at the source, at the tire-pavement interface, which accounts for over 75% of traffic noise. Tire and vehicle manufacturers have addressed the problem by offering consumers quieter tires and vehicles. Can highway engineers offer customers a quieter pavement? Quiet pavements are new to the U.S. so a technology scan team comprising federal, state, academia and industry personnel was formed by the FHWA and AASHTO to examine what has been done in Europe. 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 following is what was discovered.
For the last 20 years, the systematic reduction of noise associated with roadway operations in Europe has been a priority issue. By 2007, countries in the European Union (EU) will publish noise contour maps along all existing roadways to understand the social impact of noise. Then each country will be challenged to address problems identified in the noise map with a publicized plan of action. Communities and governments will have to work together on this quality-of-life issue. Aggressive policy directives to limit noise along newly constructed facilities exist in most countries and quiet-pavement alternatives are used as one of the tools to fix the noise problem.
The team visited Denmark, the Netherlands, France, Italy and the United Kingdom and met with some of Europe’s leading experts in acoustics, noise modeling, noise policy and quiet-pavement design. Team members were tasked to focus on one of the following topics: policy, design, noise analysis, construction, maintenance and research. The following is an executive summary written by the scan team members and the report facilitator.
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 visited have implemented policies that require consideration of quiet pavement where noise is anticipated to be a concern. Additionally, on June 25, 2002, the EU implemented a significant noise directive which requires all member countries to:
Determine exposure to environmental noise through noise mapping, including rural areas;
- Use uniform prediction methods of assessment common to the member states;
- Ensure that information on environmental noise is made available to the public; and
- Adopt action plans by the member states based upon noise-mapping results with a view toward preventing and reducing environmental noise.
The directive requires all member states to complete the strategic noise maps and adopt an action plan by June 30, 2007.
As is often the case in the U.S., the implementation budgets for the countries visited were much smaller than deemed necessary to totally implement the policy directive. 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 approximately 10-25% more than traditional surfacing.
The focus of the European effort is contained in three major quiet-pavement technologies: thin-surfaced, negatively textured gap-graded asphalt mixes; single- and double-layer highly porous asphalt mixes (greater than 18% voids); and exposed aggregate concrete 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 in rural and high-speed facilities with moderate winter conditions. Many highway projects are specified using performance specifications and are selected using best value. 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.
The source level of quiet pavements is being incorporated into existing highway noise prediction models using varying methodologies.
Harmonoise, 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 understood than for light vehicles; this topic is being investigated. Pavement noise benefits of as little as 2 dB are being utilized in integrated noise strategies.
Normal construction equipment and technology are used to construct the 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 U.S., factors other than low bid are considered when awarding pavement construction contracts. Also, a contractor warranty of at least three years is typically included in the contract.
There are still minor but persistent disagreements 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 each 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 that could be used to substantiate either claim.
Winter maintenance remains a challenge, especially on the highly porous pavements. Winter maintenance relies on advanced use of pre-wetted salt to fight formation of “black ice” on the highly porous pavements resulting in a winter maintenance cost increase of 25-50%. Some countries visited have discontinued using highly porous pavements in snow and ice regions and instead are using SMA-type pavements with small aggregate.
Perhaps the most impressive finding of the team relates to the extensive amount of research on quiet-pavement technology under way (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 Harmonize) in the countries visited. It was obvious that research is a vital part of the European culture. Much of this research is conducted in partnership between government and industry.
There are 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 then judged by a panel of topical experts. The best ideas are given permission to be 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.
The team identified a significant number of implementation recommendations that were then categorized into short-term and long-term proposals. Following are some of the recommendations for immediate implementation.
- 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 appears to have potential application on high-speed facilities and produce exceptionally quiet pavements. Thus, this system appears to merit additional evaluation and research in the U.S. Porous mixes should not be placed in urban areas where the operating speed drops below 45 mph since highly porous mixes tend to clog under slow traffic.
- 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 mm through 0/10 mm. Since most state departments of transportation 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 1-3 dB.
- Thin textured overlays using a small aggregate size are recommended for urban or low-speed sections. To achieve noise reduction, texture should always be negative (pavement depressions).
- Diamond grinding blade configurations should be investigated and optimized to enhance noise-reducing properties of existing concrete surfaces in noise-sensitive locations.
- Exposed aggregate concrete pavements should be researched further and considered when constructing new concrete pavements.
- A team of acoustical experts and pavement engineering personnel should begin the process of developing American Association of State Highway & Transportation Officials (AASHTO) protocols for measurement of the acoustical performance of quiet pavements. These protocols should capitalize on the extensive work completed and ongoing in Europe, as well as other locations in 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.
- 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.
A tire-pavement noise strategic planning workshop was held at Purdue University on Sept. 14-16, 2004, to develop a road map to solve noise problems in the U.S. As a result an expert task group has been formed and other entities such as the Transportation Research Board and the National Co-operative Highway Research Program will begin the process to develop the most suitable quiet-pavement technologies for highways in the U.S.