Disentangling distance decay from vegetation-ground effects on traffic noise attenuation in highway-adjacent urban parks

Quantifying how vegetation and ground surfaces independently contribute to traffic noise attenuation, over and above geometric spreading, is a critical gap in urban green space planning. This study explicitly separates baseline geometric spreading from excess attenuation (additional noise reduction beyond distance decay) to quantify how vegetation and ground surfaces uniquely affect sound propagation. We measured equivalent continuous sound levels along transects extending 60 m from the highway edge in six road-aligned parks. A mixed-effects model showed that noise decreased by 8.48 dB per natural log unit increase in distance (β = − 8.48, p < 0.001; marginal R 2 = 0.61, conditional R 2 = 0.73). Residual excess attenuation (observed minus distance-predicted noise) was then modeled against vegetation structure and ground surface characteristics (adjusted R 2 = 0.53). Vegetation height (β = − 0.38, p < 0.001), canopy density (β = − 0.041, p < 0.001), NDVI (β = − 3.21, p < 0.001), and shrub density (β = − 0.19, p < 0.001) were associated with greater noise reduction, whereas higher open soil proportion (β = 0.052, p < 0.001) and ground hardness (β = 0.87, p = 0.003) were associated with higher residual noise levels (i.e., less attenuation). Scenario simulations indicated that increasing vegetation height by 8 m, canopy density by 60%, shrub density by 7 stems per 10 m 2 , and reducing open soil proportion by 40% could achieve up to 10.13 dBA of additional noise reduction beyond distance decay. These findings quantify the independent contributions of vegetation and ground surfaces to excess attenuation, supporting dense, vertically complex planting designs for noise mitigation in highway-adjacent urban green spaces.