Uncertainty in the CPX method (ISO 11819-2/3) and its implications for pavement evaluation

The close-proximity method, defined in International Standard ISO 11819-2, is frequently used for assessment of the acoustic performance of pavements. Road authorities refer to this standard when defining acoustic requirements in contracts with road builders. The method is proposed as a base for the acoustic classification of pavement types currently under development in the CEN organization. This and other applications may have legal consequences in case of non-compliance. It is therefore essential that the accuracy of the method and the spread in performance of reference tyres is well understood. Even more relevant is that the interpretation of test results in terms of compliance yes/no is based on an unambiguous definition frame. In this paper the results of an analytical (according to ISO Guide to the Expression of Uncertainties in Measurements) and the statistical (according to ISO 5725) approach will be discussed and compared against each other. Recommendations for improving accuracy will be presented. Next the interpretation of test results in an approval system will be discussed in terms of true and false positives or negatives and the role of pre-knowledge will be explored.