Road traffic noise annoyance mitigation by green window view: Optimizing green quantity and quality

There is convincing real-life evidence that seeing outdoor vegetation through the windows of one's dwelling is able to mitigate negative health effects due to exposure to environmental noise, in particular for noise annoyance due to road traffic. However, design guidelines with respect to green quantity and quality to maximally benefit from this audio-visual interaction are currently lacking, but are mandatory when this idea is to be used in urban sound (and green) planning. Therefore, two virtual reality (VR) experiments were conducted, where participants were positioned near the window of a living room overlooking a city ring road, where the central reservation was used to design various greening scenarios. Participants were exposed to an A-weighted equivalent sound pressure level of 67 dB at eardrum (window partly opened). In the first experiment (79 participants), containing trees of two visually similar tree species, the optimal green quantity (using RGB greenness) was found to be near 30%. This effect, however, was not very pronounced and only amounted to 0.5 units on an 11-point noise annoyance scale. Only the very dense vegetation belt (50%) lead to a higher self-reported noise annoyance at the 5% statistical significance level. In the second VR experiment (62 other participants), vegetation quantity was fixed near this optimum, while green quality varied on the dimensions species richness, colorfulness, and maintenance degree. Green infrastructure containing most colors, or those containing most species, lead to a minimum in self-reported noise annoyance (0.7 units difference on the 11-point annoyance scale). Further analysis suggested that aesthetic value of the green infrastructure is the driving factor for the positive audio-visual interactions observed, consistent with the presumed mechanisms why green window view is able to reduce noise annoyance at home.