Explain the Mechanistic-Empirical Pavement Design Guide (MEPDG) methodology.

MEPDG uses mechanistic analysis (stress-strain under loads) with empirical transfer functions linking responses to distress. Process: (1) Define inputs - traffic spectra, climate hourly data, material properties including time/temperature dependencies, (2) Calculate pavement responses (strain, stress, deflection) using layered elastic or finite element, (3) Predict incremental damage - fatigue cracking from strain repetitions, rutting from permanent deformation, thermal cracking from cooling cycles, (4) Accumulate damage over design life (monthly/seasonal), (5) Calibrate models to local conditions. More rational than empirical methods, considers actual traffic and climate.