Explain the AASHTO method for flexible pavement design.

AASHTO 1993 flexible pavement design uses: log(W18) = ZR*So + 9.36*log(SN+1) - 0.20 + log[(delta_PSI)/(4.2-1.5)] / [0.40 + 1094/(SN+1)^5.19] + 2.32*log(MR) - 8.07. Where W18 = 18-kip ESALs, ZR = reliability factor, So = standard deviation, SN = structural number, PSI = serviceability loss, MR = resilient modulus. SN = a1D1 + a2D2m2 + a3D3m3, where a = layer coefficient, D = thickness, m = drainage factor. Design iterates to find layer thicknesses providing required SN.