Optimising New Zealand’s State highway low-noise road surface

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 L to date. Further trials are planned for early 2022 to optimise the air voids content for the preferred EPA7 surface.