Effects of geometric design of forest road and roadside vegetation on traffic noise reduction

Geometric design of forest roads and design of their landscapes can reduce noise pollution and its harmful effects on human health. We investigated the effects of technical and biological parameters such as geometric road design and various roadside tree stands on reducing noise pollution according to the tree density and distance from roadway in Darabkola Forests, Sari, Iran. We recorded the noise generated by a car (Land Rover) relative to changes in longitudinal slope, horizontal curve radius and type of road pavement. We also measured noise levels according to roadside tree density and stand type (coniferous and hardwood) in 40 rectangular plots of three widths (25, 100 and 300 m) and 50 m length that were randomly demarcated along forest roads. The changes in noise level were recorded using a decibel meter with an accuracy of +/- 1.5 dB and resolution of 0.1 dB. Noise levels were higher alongside unpaved roads than alongside paved roads. There was an inverse relationship between the measured noise level and horizontal curve radius. The rate of noise level on horizontal curve with a radius less than 30, 30-45 m and more than 45 m were 64.8, 70.8 and 75.9 dB, respectively. The noise level increased with the increasing longitudinal slope of the road. There was a significant difference between the noise level on slopes less than 3 % (67 dB) and 3-8 % (71.2 dB) in comparison with slopes greater than 8 % (77.8 dB), (p < 0.05). Pinus brutia L. reduced the noise level more (about 6 dB) in stands of 1/3 density of mixed hardwoods within 25 m from middle of the road. Careful design of geometric properties of forest roads as well as planting coniferous trees with hardwoods is a suitable solution for reducing noise pollution.