  The causal connection between congestions and velocity changes at different
locations induces various statistical features, which we identify and measure
in detail. We carry out an empirical analysis of large-scale traffic data on a
local motorway network around the Breitscheid intersection in the North
Rhine-Westphalia, Germany. We put forward a response function which measures
the velocity change at a certain location versus time conditioned on a
congestion at another location. We use a novel definition of the corresponding
congestion indicator to ensure causality. We find that the response of
velocities to the congestion exhibits phase changes in time. A negative
response at smaller time lags transforms into positive one at larger time lags,
implying a certain traffic mechanism. The response decays as a power law with
the distance. We also identify a scaling property leading to a collapse of the
response functions on one curve.
