A longer design life for flexible pavements, which carry theheaviest volumes of traffic, will yield a lower whole life cost.This will require strategies for design that decrease the needfor maintenance and thereby cause less disruption to the roaduser.

This paper reviews the current philosophy and criteria fordesign and considers information on the performance of roads thathas been collected since the last revision of UK design standards,in 1984. This has demonstrated that the deterioration of thick,well-constructed, fully-flexible pavements is not structural,and that deterioration generally occurs at the surface in theform of cracking and rutting. The evidence suggests that fatigueand structural deformation originating deep within the pavementstructure are not the prevalent modes of deterioration. It alsoshows that changes that occur to the structural properties ofthe bituminous materials over the life of the road are crucialto the understanding of its behaviour. These changes can helpto explain why conventional mechanisms of deterioration do notoccur. They imply that a road built above a minimum strength willremain structurally serviceable for a considerable period, providedthat non-structural deterioration in the form of cracks and deformationare detected and remedied before they have a serious impact onthe structural integrity of the road.