Evolution of Pavement Design Methods

Pavement design methods that are currently available can be broadly divided into three groups:

Type	Approach	Examples
Empirical	Use empirical equations or charts to correlate structure, material, traffic, and climate characteristics to pavement performance.	Caltrans R-value method for new flexible pavement design (1937-1964), Caltrans overlay design method (1951-1980)
Classical Mechanistic-Empirical	Determining pavement responses due to loading through mathematical models and relating those responses to pavement performances	Shell pavement design manual (1978), Asphalt Institute method (1982), AASHTO guide for design of pavement structures (1986, 1993)
Mechanistic-Empirical	Same as classical mechanistic-empirical method, except each distress is explicitly accounted for and the empirical part for each distress requires local calibration	AASHTO Mechanistic Empirical Pavement Design Guide (2004-present) California Mechanistic-Empirical Design (CalME) (2011-present)

Although one can argue that there is a fourth group called "Mechanistic" method, which goes a step further from the Mechanistic-Empirical method in terms of reducing the role of the empirical calibration. Specifically, the empirical data is only needed for validation because the mechanistic model is expected to capture all the behaviors affecting pavement performance. As of this writing, there is no mechanistic pavement design method that is used in routine design.