Longitudinal variability in tyre/road noise is often associated with low-noise porous asphalt surfaces, with CPX testing results for individual 20-metre road segments commonly varying by up to 6 dB along new projects in New Zealand. In November 2018, following on from previous trials investigating the effects of air voids and stone size, three EPA7 trial sections were constructed to investigate the effects of layer thickness on tyre/road noise. The thickness effect was found to be approximately –2 dB LCPX:P1,80 per 10 mm increase in target layer thickness across the three trial sections. Core samples taken from a new road showed EPA7 layer thicknesses that differed from the target layer thickness by up to 15 mm. These findings suggest that variations in layer thickness may be a key contributor to longitudinal variations in tyre/road noise on roading projects using porous asphalt. A further investigation involving a detailed thickness survey is currently underway to better understand layer thickness variability and its effect on tyre/road noise.

Since 2017, several porous asphalt trials have been built in New Zealand in order to investigate the effects of air voids, stone size and layer thickness on tyre/road noise. These trials have shown that all three variables play an important role in noise from porous asphalt surfaces. The trial results have led to a 40 mm thick EPA7 surface (porous asphalt with 7 mm nominal stone size) becoming the preferred low-noise surface for use on New Zealand State highways, with a slightly thicker 50 mm version being preferred in particularly noise-sensitive areas. Regular CPX testing is being conducted to monitor the long-term acoustic behaviour of the surfaces, and this testing has not shown any significant changes in LCPX:P1,80 to date. Further trials are planned for early 2022 to optimise the air voids content for the preferred EPA7 surface.