The context is relatively simple...this is the way it is treated in the US for the AASHTO Design Method...

Determine the amount of traffic and its distribution by weight (equivalent single axle loads (ESAL))

Determine the structural number (SN) required for that number of ESALs over the design period (usually 10 to 20 years)

Mix and match combinations of structural layers (asphalt, graded aggregate base, stabilization, subgrade) to satisfy the relationship of:

SN=a1D1+a2D2m+a3D3m...anDnm

where, a=structural layer coefficient for each material
       D=layer thickness
       m=moisture effect of a particular layer

There are numerous texts that outline this procedure and provide the tables for the various values, as well as a nomograph to assist the design.  The most significant reference for the AASHTO method is the AASHTO Design Manual for Pavement Structures.

Thanks Ron for the immediate response!

As a corollary to Ron's post, think DRAINAGE - DRAINAGE - DRAINAGE.  See Cedergren's classic book on seepage and flow nets - he has a section on pavement drainage.  In the past, pavement designers seemed to feel that they could design a pavement "strong enough" not to worry about water - but in the end, it doesn't work all that well.