Beckmann, McGuire, and Winsten (1956) were the first to rigorously formulate these conditions
mathematically. Specifically, Beckmann, McGuire, and Winsten (1956) established
the equivalence between the transportation network equilibrium conditions, which state that
all used paths connecting an origin/destination (O/D) pair will have equal and minimal
travel times (or costs) (corresponding to Wardrop’s first principle), and the Kuhn-Tucker
(1951) conditions of an appropriately constructed optimization problem, under a symmetry
assumption on the underlying functions. Hence, in this case, the equilibrium link and path
flows could be obtained as the solution of a mathematical programming problem. Their
approach made the formulation, analysis, and subsequent computation of solutions to transportation
network problems based on actual transportation networks realizable.
Dafermos and Sparrow (1969) coined the terms user-optimized (U-O) and system-optimized
(S-O) transportation networks to distinguish between two distinct situations in which, respectively,
users act unilaterally, in their own self-interest, in selecting their routes, and in
which users select routes according to what is optimal from a societal point of view, in that
the total cost in the network system is minimized. In the latter problem, marginal total costs
rather than average costs are equilibrated. The former problem coincides with Wardrop’s
first principle, and the latter with Wardrop’s second principle. See Table 1 for the two
distinct behavioral principles underlying transportation networks.