If you’ve ever walked near a busy highway, you’re aware of how loud roadways can be. If you’ve ever lived near a busy highway, you know firsthand that highway noise can be a real nuisance. In this edition of the RoadReady Newsletter, we’re going to take a look at roadway noise and how it can be mitigated.
Worldwide, transportation noise is recognized as the source of noise that is complained about the most. Roadway noise has adverse impacts on human health, human comfort, and the environment. Because of the huge volume of roadways in the U.S., it is estimated that 19% of the land area of the U.S. is ecologically impacted by roads. This makes any roadway characteristic such as traffic noise have potentially far reaching consequences. As a result, more and more roadway agencies are looking towards quiet pavements – pavements which reduce the noise of the tire-pavement interaction through a number of different strategies- to mitigate these issues in critical areas.
Effects of Roadway Noise
At 55 mph, a typical passenger vehicle produces noise in the neighborhood of 73 decibels, while a large truck can reach 86 decibels. These numbers are in the range that causes human discomfort, and at the upper limits are capable of causing hearing damage with extended exposure. Highway noise has been shown to adversely affect a number of different animal species as well. These effects spread far beyond the physical footprint of the roadway. For instance, research on small birds has shown avoidance of areas as a function of highway noise up to three kilometers from the actual highway. Continued research is exploring the particular effects of highway noise on different species to more fully understand these impacts.
Components of Roadway Noise
Traffic noise is made up of several components. These include engine noise, noise from air movement, exhaust noise, and noise from tire-pavement contact. At high speeds, the tire-pavement interaction makes up the bulk of traffic noise. This is generally the source that is targeted for reduction.
Making a Pavement “Quiet”
A number of techniques are used to limit the noise generated from the tire-pavement interaction. Such strategies exist for both rigid and flexible pavement types.
Strategies for quiet flexible pavements include:
- Open-graded asphalt pavements – Open graded asphalt pavements use aggregate blends with a small percentage of particles in the fine range to create a high volume of air voids. These pavements both limit the amount of noise produced as well as increasing the amount of noise absorbed by the pavement. These pavements can achieve a reduction of highway noise from three to five decibels.
- Stone matrix asphalt pavements – Stone matrix asphalt pavements use gap-graded aggregate blends and rely more heavily on aggregate interaction for structural support. The coarse surface texture of these pavements reduces pavement noise on a similar magnitude to open-graded asphalt.
- Rubberized asphalt pavements – Rubberized asphalt incorporates ground rubber into the asphalt mix, usually from recycled tires. Studies in Arizona have shown an average noise reduction of 4 decibels, on par with the reduction of the above pavement types.
Strategies for rigid pavements include:
- Open-graded aggregate - Like flexible pavements, using an aggregate blend with an open gradation in rigid pavements helps to reduce highway noise.
- Surface Texturing– Selecting the correct type of surface texturing can impact the noise output of rigid pavements. Use of diamond grinding rather than longitudinal or transverse tining can reduce traffic noise by three to five decibels. However, anticipation of diamond grinding can require design of thicker concrete pavements to account for the loss of material from each grinding operation.
Alternate Strategy for Noise Reduction
Limiting tire noise is not the only option for keeping highway noise levels in check. Construction of noise-walls around major highways is a commonly used solution. While noise-walls provide effective reduction, they become less efficient in areas that are not directly behind the wall such as hillsides. In addition, they provide no increase in ride quality for the drivers on the roadway itself. Finally, noise walls are typically are a more expensive solution than altering pavement design. However, in cases where alternative pavement design is not sufficient or is precluded by other design needs, noise walls are a useful alternative.
Keeping it Quiet
Pavement noise can be reduced in a number of ways and in a number of settings. As a result, noise mitigation is a worthy consideration on any project with high enough speeds to create an audible nuisance, or more importantly actual health risks. If pavement noise is a problem on a given project, there is most likely a way that it can be solved. Giving proper attention to noise issues creates a better roadway environment for users, pedestrians, residents, and wildlife.