  Daganzo's criticisms of second-order fluid approximations of traffic flow [C.
Daganzo, Transpn. Res. B. 29, 277-286 (1995)] and Aw and Rascle's proposal how
to overcome them [A. Aw and M. Rascle, SIAM J. Appl. Math. 60, 916-938 (2000)]
have stimulated an intensive scientific activity in the field of traffic
modeling. Here, we will revisit their arguments and the interpretations behind
them. We will start by analyzing the linear stability of traffic models, which
is a widely established approach to study the ability of traffic models to
describe emergent traffic jams. Besides deriving a collection of useful
formulas for stability analyses, the main attention is put on the
characteristic speeds, which are related to the group velocities of the
linearized model equations. Most macroscopic traffic models with a dynamic
velocity equation appear to predict two characteristic speeds, one of which is
faster than the average velocity. This has been claimed to constitute a
theoretical inconsistency. We will carefully discuss arguments for and against
this view. In particular, we will shed some new light on the problem by
comparing Payne's macroscopic traffic model with the Aw-Rascle model and
macroscopic with microscopic traffic models.
