Although these relationships are accepted by most leading authorities and are
referred to in almost every reference on the subject, it is important to understand that
they make many assumptions about physical quantities that are seldom consistent
throughout a length of pavement. For instance, the friction factor can be affected by
something as insignificant as a large footprint in the base course prior to paving.
Additionally, the environment can play an important role as water and salt erode the
steel, thus reducing the sectional area of the steel.