We study critical threshold phenomena in a dynamic continuum traffic flow model known
as the Payne and Whitham (PW) model.
This model is a quasi-linear hyperbolic relaxation system, and when equilibrium velocity
is specifically associated with pressure, the equilibrium characteristic speed resonates with one characteristic speed of the full relaxation system.
For a scenario of physical interest we identify a lower threshold for
finite time singularity in solutions and an upper threshold for the global existence of the smooth solution.
The set of initial data leading to global smooth solutions is large, in particular allowing
initial velocity of negative slope.