  Traffic flow at low densities (free traffic) is characterized by a
quasi-one-dimensional relation between traffic flow and vehicle density, while
no such fundamental diagram exists for `synchronized' congested traffic flow.
Instead, a two-dimensional area of widely scattered flow-density data is
observed as a consequence of a complex traffic dynamics. For an explanation of
this phenomenon and transitions between the different traffic phases, we
propose a new class of molecular-dynamics-like, microscopic traffic models
based on times to collisions and discuss the properties by means of analytical
arguments. Similar models may help to understand the laminar and turbulent
phases in the dynamics of stock markets as well as the transitions among them.
