  We investigate the adaptation of the time headways in car-following models as
a function of the local velocity variance, which is a measure of the
inhomogeneity of traffic flow. We apply this mechanism to several car-following
models and simulate traffic breakdowns in open systems with an on-ramp as
bottleneck. Single-vehicle data generated by several 'virtual detectors' show a
semi-quantitative agreement with microscopic data from the Dutch freeway A9.
This includes the observed distributions of the net time headways and
times-to-collision for free and congested traffic. While the times-to-collision
show a nearly universal distribution in free and congested traffic, the modal
value of the time headway distribution is shifted by a factor of about two in
congested conditions. Macroscopically, this corresponds to the 'capacity drop'
at the transition from free to congested traffic. Finally, we explain the wide
scattering of one-minute flow-density data by a self-organized variance-driven
process that leads to the spontaneous formation and decay of long-lived
platoons even for deterministic dynamics on a single lane.
