  Based on an empirical study of real field traffic data measured in 1996--2014
through road detectors installed on German freeways, we reveal physical
features of empirical nuclei for spontaneous traffic breakdown in free flow at
highway bottlenecks. It is shown that the source of a nucleus for traffic
breakdown is the solely difference between empirical spontaneous and induced
traffic breakdowns at a highway bottleneck. Microscopic traffic simulations
with a stochastic traffic flow model in the framework of three-phase theory
explain the empirical findings. It turns out that in the most cases, a nucleus
for empirical spontaneous traffic breakdown occurs through an interaction of
one of waves in free flow with an empirical permanent speed disturbance
localized at a highway bottleneck. The wave is a localized structure in free
flow, in which the total flow rate is larger and the speed averaged across the
highway is smaller than outside the wave. The waves in free flow appear due to
oscilations in the percentage of slow vehicles; these waves propagate with the
average speed of slow vehicles in free flow (about 85--88 km/h for German
highways). Any of the waves exhibits a two-dimensional asymmetric
spatiotemporal structure: Wave's characteristics are different in different
highway lanes.
