  We study the traffic flow on a highway with ramps through numerical
simulations of a hydrodynamic traffic flow model. It is found that the presence
of the external vehicle flux through ramps generates a new state of recurring
humps (RH). This novel dynamic state is characterized by temporal oscillations
of the vehicle density and velocity which are localized near ramps, and found
to be the origin of the synchronized traffic flow reported recently [PRL 79,
4030 (1997)]. We also argue that the dynamic phase transitions between the free
flow and the RH state can be interpreted as a subcritical Hopf bifurcation.
