Recent and some new results of observations of traffic flow dynamics in congested conditions are considered. On the basis of these results, hypotheses about properties of congested flow are proposed and discussed. In particular, it is proposed that the complexity of congested flow is linked to (a) the existence of three qualitatively different phases in traffic flow: free flow, synchronized flow, and wide traffic jams; (b) an occurrence of three qualitatively different types of phase transitions between these three phases of traffic flow; (c) the result that all these qualitatively different phase transitions are related to the same class of "first-order local phase transitions"; (d) complex dynamical behavior of synchronized flow; and (e) the existence of two qualitatively different kinds of "nucleation effects" in traffic flow: the nucleation effect responsible for the jam's formation, which is linked to an avalanche increase in the amplitude of an initial local perturbation of traffic variables (vehicle speed, density, or both), and the one responsible for the phase transition from free flow to synchronized flow, which is linked to an avalanche decrease in the probability of passing in free traffic flow.