Tyre/road noise is the dominant source of noise from high-speed traffic and, therefore, considerable benefits could be achieved by greater use of surfaces that produce lower levels of tyre noise. Currently, the most widely used test method available for assessing the noise from different road surfaces is the Statistical Pass-by (SPB) technique. However, there are some disadvantages with the SPB method. For example, the results are not solely dependent on the road surface characteristics but also depend upon the type of vehicles selected from the traffic stream during measurements. To overcome some of these difficulties, a supplementary method, known as the Close-proximity (CPX) method is currently under development. This type of measurement attempts to isolate the noise specifically from the tyre/surface and therefore overcomes some of the potential difficulties encountered with the SPB method. This report provides a critical overview of the two methods and examines the degree of correlation between the noise levels derived from each procedure. The ability to estimate SPB traffic noise levels from CPX data would be of particular value in relating the acoustic characteristics of road surfaces with roadside traffic noise levels and, hence, on community noise impact. The greater efficiency of the CPX method would potentially allow large sections of the road network to be assessed in this manner.

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