Pavement designs must account for various failure possibilities as well as uncertainties in environmental conditions. For example, pavements can be subjected to a number of detrimental effects from traffic loads, including fatigue failures (cracking), which are the result of the accumulating effects of traffic passing over the pavement surface. The pavement is also placed in an uncontrolled environment that produces temperature extremes and moisture variations. In addition, pavement structures can have considerable variability in material quality (materials are often drawn from local sources due to the expense of transportation) and construction quality. The combination of the environment, traffic loads, material variations, and construction variations requires a robust design procedure. The AASHTO pavement design procedure accounts for most of the critical elements needed for an effective pavement design. It considers the environment, traffic loads, and material strengths in a methodology that is relatively easy to use. The AASHTO pavement design procedure has been widely accepted throughout the United States and around the world.