The Equilibrium assignment distributes the demand according to Wardrop's first principle.
"Every road user selects his route in such a way, that the impedance on all alternative routes is the same, and that switching to a different route would increase personal travel time (user optimum)."
This behavioral hypothesis underlies the unrealistic assumption that every road user is fully informed about the network state. In transport planning this hypothesis is approved of given a fundamental methodical advantage of the equilibrium assignment - with quite general requirements, the existence and uniqueness of the assignment result (expressed in volumes of the network object) is guaranteed. Moreover, measures for the distance of an approximation solution from the equilibrium exist, from which an objective termination criterion can be derived for the procedure, which generally is an iterative problem solution.
The equilibrium assignment determines a user optimum which differs from a system optimum, as shown in Table 133 and Table 134.
- A user optimum means that the same impedance results for all routes of a traffic relation between zones i and j (within the scope of calculation accuracy). This results directly from the condition, that changing to another route is not profitable for any road user (Table 133).
- A system optimum, however, means that the total impedance in the network, which is the product of route impedance and route volume, is minimized for all OD pairs. On average, this procedure leads to shorter journey times per road user, but there are (few) road users which use routes to serve the general public, with an impedance above average (Table 134).
