Influence of Various Material and Traffic Inputs on Flexible Pavement Design Methods for Alabama Roadways

Statistical analyses of over 2,000 pavement designs using the 1993 American Association of State Highway Transportation Officials Design Guide was conducted. Variations in traffic levels, and subgrade, base, and hot mix asphalt (HMA) moduli and thickness showed that the only significant material and traffic design inputs were the subgrade modulus and the initial ESAL loads. A plus or minus 5% change in the design traffic level did not statistically influence the determination of the structural number (SN). A subgrade modulus of $\mathrm{3,000}\mathrm{psi}$ and a linear growth rate of 4% for roadways with annual average daily traffic of more than 10,000 is recommended for determining the SN. The growth rate for lower traffic volume roadways will vary significantly between 0 and 3%. A more limited evaluation of the 2002 Design Guide showed that a mechanistic–empirical method of flexible pavement design is strongly influenced by the HMA thickness and the distribution of truck types within the annual average daily traffic. Selecting the appropriate roadway functional classification defines the default percent of traffic that is comprised of trucks and is important to the program output. Performance data for two Alabama long-term pavement performance sections was used to verify both design methods.