In this paper we study the dynamic user-equilibrium problem. The development of Intelligent Vehicle Highway Systems (IVHS) has made this problem very popular in the recent years. In this paper we take a hydrodynamic theory approach. Hydrodynamic theory provides us with a set of laws that describe the dynamics governing the problem and give rise to a general model. Our goals in this paper are: (i) using these laws to propose and study a model for the problem and simpler forms that arise. (ii) through our model to propose travel time functions for the problem and (iii) to propose methods for solving our model in the disjoint path network case. In this last part we propose a dynamic version of the equilibration algorithm and an algorithm which speeds it up and is particularly efficient for large scale networks.