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The risk involved in pavement design is that the pavement life will be shorter than the design life. While the literature reveals that statistical methods can be used to estimate the risk (or reliability) of a pavement design, the researchers do not appear to have demonstrated the rigour of their analysis by comparing their results with the performance of actual pavements.

This research project, carried out in 2008 on four state highway networks in New Zealand, studied how the interaction of all the variables relating to pavement life combine to influence pavement performance.

The probabilities of failures were investigated through the available RAMM data. The study examined the rutting and roughness performance of unbound granular pavement and full-depth asphalt pavement.

Based on these findings, it is proposed that thin-surfaced granular pavements have a bimodal distribution of life. The first peak is in the first one to two years, when shallow shear and potholing can occur. After this period, the pavement settles down and the average life will approximate 45 50 years under moderate traffic. It is also concluded that although the pavements have not been failing through rutting or roughness, the Austroads Pavement design guides' proposed risk of a 5% probability of not achieving the pavement's design life appears to be correct.

Keywords: age, failure, life, maintenance cost, New Zealand, pavements, pavement design, performance, risk, roughness, rutsKeywords: age, failure, life, maintenance cost, New Zealand, pavements, pavement design, performance, risk, roughness, ruts