The objective function given by (6) is simply a device constructed to obtain a solution
using general purpose convex programming algorithms. It does not possess the economic
meaning of the objective function encountered in the system-optimization problem which will
be recalled below. Note that in the case of separable, as well as nonseparable, but symmetric
(which we come back to later), user link cost functions, the λω term in (5) corresponds to
the Lagrange multiplier associated with the constraint (7) for that O/D pair ω. However, in
the case of nonseparable and asymmetric functions there is no optimization reformulation of
the transportation network equilibrium conditions (5) and the λω term simply reflects the
minimum user cost associated with the O/D pair ω at the equilibrium. As noted as early as
Dafermos and Sparrow (1969), the above network equilibrium conditions also correspond to
a Nash equilibrium (see Nash (1950, 1951)). This connection has now garnered great interest
in computer science (see Roughgarden (2005) and the references therein). The equilibrium
link flow pattern is unique for problem (6), subject to (7) – (9), if the objective function (6)
is strictly convex (for additional background on optimization theory, see Bazaraa, Sherali,
and Shetty (1993)).