Effect of traffic noise on perceived visual impact of motorway traffic

• Visual impact increases by traffic volume in both sound conditions.
• Visual impact is higher in natural landscape in both sound conditions.
• Decrease of visual impact by distance is less rapid and less clear with sound.
• Traffic noise raises visual impact by largely constant levels in various scenarios.

Visual impact is one of the major environmental impacts of motorways and requires adequate assessment. This study investigated the effect of traffic noise on the perceived visual impact of motorway traffic by comparing impact with sound to impact without sound. Computer visualisation and edited audio recordings were used to simulate different traffic and landscape scenarios, varying in four traffic conditions, two types of landscape, and three viewing distances. Subjective visual judgments on the simulated scenes with and without sound were obtained in a laboratory experiment. The results show that motorway traffic induced significant visual impact. In both sound conditions, increases in traffic volume led to higher visual impact and changes in traffic composition changed the impact significantly when traffic flow was low. Visual impact was significantly higher in the natural landscape and the increment was largely constant and independent from the effect of traffic condition in both sound conditions. The effect of viewing distance was also significant and there was a rapid-to-gentle decrease of visual impact by distance both with and without sound, but the decrease with sound was less rapid and the decrease pattern less clear. Overall, introduction of traffic noise increased the visual impact by a largely constant level which did not show clear dependence with noise level, traffic condition, landscape type, or viewing distance, although there was a possible effect of viewing distance on the increase. It suggests that the additional impact caused by traffic noise should be considered in visual impact assessment of motorway projects.