Now these plans need to be executed. These simulations come at many different levels of resolution and fidelity, reaching from the traditional steady-state flow-based cost function to very detailed micro-simulations.
If one is interested in time-dependent results, as for example the queue built-up during the onset of rush periods, the simulation needs to be sufficiently realistic to contain such dynamics. Traditional flow-based cost functions are not able to realistically deal with such dynamical effects, at least not in a straightforward way. Thus, the right simulation has to be chosen according to what aspects of the dynamics one wants to have represented for a given question.