Why Is Traffic Noise On Roads Louder When It Is Raining? - New Scientist

Why is the noise from traffic on roads louder when it is raining or the road is wet than it is on a dry day?

Elisabete Freitas University of Minho, Guimarães, Portugal

Road traffic noise is mainly caused by tyre vibrations due to the friction between the tyre and the road surface. This noise is amplified by the horn-like shape in front of and behind where each tyre touches the road – known as the “horn effect”.

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When it is raining, there is an obvious increase in the traffic noise due to the impact of the rain on the vehicles and the spray caused by the movement of the wheels. As well as this, the water on the surface interferes with the generation of noise caused by the contact between road and tyre, and how this noise is propagated.

Our studies on a Portuguese motorway found that the presence of water increases the overall traffic noise by 4 decibels. This increase is mainly at frequencies greater than 1000 hertz (which is at the middle of the frequency range for human hearing), where it can reach 15 decibels.

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This noise increase depends on the type of tyre, the material the road is made from, the type of vehicle and the speed at which it is travelling. Porous road surfaces (such as special asphalt) are often less noisy than non-porous ones, but when these surfaces are wet, the traffic noise increases by similar amounts on both.

The difference between dry and wet traffic noise is smaller for vehicles travelling at higher speeds and for heavier vehicles.

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