In practice, the common behavioral assumption is that travelers choose the available
route having the least travel time between their origin and destination (O-D), reflecting the idea
that travel is rarely a goal in and of itself, but instead involves some time, cost, or disutility that
travelers would prefer to avoid. (More generally, travelers are sometimes assumed to choose the
route having the lowest cost, or the minimum disutility, appropriately defined. Within limits, the
particular impedance variable assumed to influence route choice does not affect the discussion
here, so we will usually just say that travelers minimize their travel time with the understanding
that the discussion could apply equally well to travel cost or other disutility measures.) Due to
congestion effects, the travel time of a route between an O-D also depends on the choices made
by other travelers, who are themselves also trying to choose the least travel time route between
their own O-D. When every traveler succeeds in finding such a route, every used route has the
minimum time or cost between O-D; moreover, for each O-D pair, every route used has the same
travel time. This condition is known as user equilibrium.