  It is typically assumed that the fluctuations associated with a stationary
broadband incoherent field propagate in free space at the speed of light in
vacuum c. Here we introduce the concept of 'coherence group velocity', which --
in analogy to the group velocity of coherent pulses -- is the speed of the peak
of the coherence function. We confirm experimentally that incorporating a
judicious spatio-temporal spectral structure into a field allows tuning its
coherence group velocity in free space. Utilizing light from a
super-luminescent diode, we interferometrically measure the group delay
encountered by the cross-correlation of a structured field synthesized from
this source with the unstructured diode field. By tracking the propagation of
this cross-correlation function, we measure coherence group velocities in the
range from 12c to -6c.
