Low noise road surfaces
Road surface has a significant effect on the level of rolling noise generated as a result of the interaction between vehicle tyres and the road surface. This type of noise is referred to as ‘tyre/road noise’. The level of tyre/road noise generated is dependent on the mix and the speed of the vehicles. Tyre/road noise dominates other sources of noise for light vehicles at speeds between 30 and 50 km/h and for heavy vehicles at speeds between 40 and 80 km/h.
The design and construction of a road surface affect both the generation and propagation of tyre/road noise. The primary design and construction factors affecting the level of tyre/road noise are the roughness or texture, the texture pattern, and the porosity of the surface structure.
In New Zealand, tyre/road noise levels are measured relative to the performance of ‘Asphaltic Concrete (AC)’ type surfaces. The most common low noise road surface in use is ‘Open Graded Porous Asphalt (OGPA)’, which is generally about 2 dB quieter than the AC type surfaces. By contrast, the noisiest chip seal road surfaces can be up to 6 dB louder than the AC type surfaces.
The following table provides more detail about the relative noise performance of different road surfaces used in New Zealand for a 50 km/hr speed (from LTNZ Research Report 326).
| Surface | Cars | Heavy vehicles |
|---|---|---|
| Asphaltic concrete | 0 dB | 0 dB |
| OGPA-14, 20% voids | 0 dB | -2 dB |
| OGPA-14, 30% voids | -2 dB | -3 dB |
| OGPA 70mm double-layer | -2 dB | -4 dB |
| OGPA-25 | 3.5 dB | -1 dB |
| Capeseal #2/type 3 (coarse) | 3 dB | 1 dB |
| Capeseal #3/type 2 (general) | 2 dB | -1 dB |
| Capeseal #4/type 1 (fine) | 0 dB | -1 dB |
| Sealed mastic asphalt | 1.5 dB | -1.5 dB |
| Slurry seal type 3 (coarse) | 1 dB | -1 dB |
| Macadam | 3 dB | 0 dB |
| Chipseal grade 6 (smallest chip) | 3 dB | -2 dB |
| Chipseal grade 5 | 3 dB | -2 dB |
| Chipseal grade 4 | 3 dB | -2 dB |
| Chipseal grade 3 | 4 dB | 1 dB |
| Chipseal grade 2 (largest chip) | 6 dB | 1 dB |
| Two-coat seal grade 4/6 | 5 dB | 1 dB |
| Two-coat seal grade 3/5 | 6 dB | 1 dB |
| Two-coat seal grade 3/6 | 6 dB | 1 dB |
| Two-coat seal grade 2/4 | 6 dB | 1 dB |
The noise level of a particular road surface may vary with time. For example, older chip seal surfaces tend to become smoother as they wear, resulting in a slightly lower noise level relative to a newly laid surface despite having the same design standard.
Road surface decisions must also consider safety, longevity, wear, and cost.