Annex 3B: Guidelines for Asphalt Pavement 451 FLEXIBLE PAVEMENT DESIGN PROCEDURE Background There are several flexible (bituminous) pavement design procedures available, which have been field verified and used by road agencies around the world. The selection of one procedure over another is usually based on the road agency’s familiarity, exposure and satisfaction with the design results. A widely accepted flexible-pavement design procedure is discussed in the “AASTHTO Guide for Design of Pavement Structure,” which was first published by the American Association of State Highway and Transportation Officials (AASHTO) in 1972, with the latest revisions in 1993. The 1993 AASHTO design procedure is similar to the earlier version (version 1986) except for the new section on overlay designs. A revised procedure, using mechanistic empirical approach, was developed in 2002 by the US Federal Highway Authority and intended as a future revision of the 1993 AASHTO guide. In the Philippines, the current practice is based on the 1993 version of the AASHTO pavement design guide. Test data, used for the development of the design procedure for flexible pavements, were collected at the American Association of State Highway Officials (ASSHO) Road Tests in Illinois from 1958 to 1960 . AASHO was the predecessor of the current AASHTO. It is emphasized that the procedure developed by AASHTO and presented in this Appendix relates to sealed flexible pavements and should not be applied to unbound aggregate surfaced roads. The reasons are explained in the following discussions of the correlation analysis performed by AASHO, which became the basis of the design procedure. The AASHO Road Test The damage that vehicles impose on road layers depends strongly on the axle loads of the vehicles. For pavement design purposes, the damaging power of axles is related to a `standard’ axle of 8.2 metric tons using equivalence factors which have been derived primarily