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Elements of Highway Traffic Noise
Figure 2. Elements of highway traffic noise.
The interaction between tires and pavement is complex. The generated noise level is highly dependent on the road sur- face, the tire tread pattern, and construction. For the road surface, there are two broad categories of pavement: flex- ible (asphaltic concrete, aka “asphalt”) and rigid (cement concrete, aka “concrete” or “PCC” for Portland cement con- crete). Asphalt pavements vary by stone size (aka “aggregate size” or “chipping size”) and porosity; smaller stone size and higher porosity produce quieter pavements. Concrete pave- ments can vary by surface texture; negative and shallow textures produce quieter pavements. Figure 3 shows an ex- ample of a quieter pavement overlay (an asphalt rubberized friction course) on top of a loud pavement base (transversely tined concrete). Besides maintaining proper skid resistance for safety, one of the main challenges is maintaining acoustic durability for quieter pavements, particularly in areas with winter weather concerns. Also, the effectiveness of pave- ments in reducing noise is influenced by temperature: warm- er is quieter. Many researchers and practitioners are working toward recommendations and policies that allow routine use of quieter pavements that are both safe and durable.
Figure 3. Quieter pavement overlaying louder pavement, Arizona Quiet Pavement Pilot Program. From Donavan et al., 2012.
Regarding tires, one of the main influential factors is the tread pattern. There are indicators that larger groove width, angled groove, addition of circumferential grooves to a transverse pattern, and tread randomization are tread pat- tern parameters that result in lower noise levels (Sandberg and Ejsmont, 2002).
The noise levels and spectral content of highway traffic noise are influenced by vehicle type, volume, and speed as well as pavement type. The spectral content for passenger vehicles typically peaks and is dominated by frequencies around 1,000 Hz. Spectral content for heavy trucks typically peaks and is dominated by frequencies from 500 to 1,000 Hz. Trucks are much louder than passenger cars: one truck can be as loud as 10 passenger cars combined! The percentage of heavy trucks in the traffic mix can have a strong effect on sound levels in adjacent communities. The benefit of quieter pavements can be less for heavy trucks than cars, one rea- son being that the lower frequency content for trucks (e.g., around 500 Hz) is not reduced as effectively as some of the other peak frequencies (e.g., around 1,000 Hz; Rochat and Read, 2009).
Figure 4 shows the spectral content for both cars and heavy trucks, where the most dominant frequencies can be seen. It also shows how the spectral content can change for dif- ferent types of pavement and different thicknesses of pave- ment. Dense-graded asphalt (DGAC) is a standard asphalt pavement, and open-graded asphalt (OGAC) and rubber- ized asphalt (RAC) are quieter asphalt pavements. In Figure 4, it can be seen that sound energy at dominant frequencies of 630 Hz and above are effectively reduced by the quieter pavements.
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