AASHTO Flexible Pavement Design Equation Study

A revised flexible pavement design equation has recently been published by the American Association of State Highway and Transportation Officials. The revised equation contains new input parameters: resilient modulus, $M_R$, to characterize subgrade support; reliability, R to allow the designer to introduce the concept of risk analysis into the design process; and standard error, $S_o$, to represent variability in design input values. This paper summarizes the results of a significance study performed using the revised flexible pavement design equation. The study demonstrates the relative importance of design equation input parameters and provides insights into the relative change in required pavement structure thickness resulting from variations or errors in input parameter values. Study results show that variation in subgrade resilient modulus, $M_R$, has the most pronounced effect on thickness structural number, SN. Variation in the predicted number of 18-kip equivalent single-axle loads, W₁₈, has a lesser effect, and variation in combined standard error, $S_o$, has a minimal effect on $S_n$. Selection of the highest values for design reliability, R, results in significant increases in Sn.