Rarely does the actual traffic loading to failure equal the predicted traffic loading; the
difference is due to the deviations that exist. These deviations include (1) lack of fit of
the AASHTO design equations, since these are empirical equations; (2) variations in
construction, which cause variations in the equation input factors such as the strength
and thickness of pavement layers; and (3) variations in the predicted traffic (see App. EE
of Vol. 2 of the AASHTO Guide for Design of Pavement Structures, August 1986 edition).
The AASHTO equations account for these variations by multiplying the predicted
traffic by a safety factor. The safety factor is determined by the reliability desired and