The traditionally (and currently) most important method for transportation planning is Static Assignment. As said in Sec. 27.7, from our point of view a better word might be Steady State Assignment, since the assumption is that one has constant traffic streams. In fact, the model is very similar to steady state current calculations for electricity or water, where electrons or water molecules enter the system at certain points and are removed at certain other points. The main difference is that for traffic the particles have destinations which they need to reach, which means that in traffic we cannot exchange particles.
This is an extremely basic introduction into static assignment. An introduction at the same level, but with much more material in particular with respect to the history of static assignment, can be found in (92). A comprehensive but still didactic treatment is in (108).